Understanding Dates: Five common mistakes to avoid

Understanding Dates: Five common mistakes to avoid

This post was written by Laurence Harris, Head of Genealogy UK at MyHeritage.

It is important to record key events of our ancestors, including the date when each event occurred.

Usually several sources indicate an event’s date. For example, for a death: the date may be indicated on a death certificate, a headstone, a newspaper obituary and in a Grant of Probate (which authorizes distribution of a deceased person’s estate). However, those dates would have been documented using the calendar and recording conventions of the geographical location and time when the event originally took place, rather than the calendar and conventions with which today’s researcher would be familiar. Failure to take into account the original context of an event or document often results in mistakes in understanding when an event actually happened.

Here are five of the most common mistakes that can occur in interpreting dates, together with suggestions as to how these mistakes can be avoided or corrected.

1. Mixing up American and English date formats

“Elizabeth Green was born on 12.11.1904.” Family historians in the US are likely to interpret this date as December 11, 1904. However those in the UK are likely to interpret this date as 12 November 1904. Only one is correct! The original record and where any subsequent transcription took place will help to determine if this was a December or November birth. Look for nearby transcriptions in the same document to see how other dates are recorded. If you spot another date recorded as 7.13.1925 then the 13 must be a day number (as month number 13 does not exist) so the American convention of mm.dd.yyyy had been used. Therefore, Elizabeth Green was born December 11, 1904. However, if you see another date like 13.7.1925 then you can assume that the first number is the day, and that Elizabeth  was born on 13 November 1904.

To avoid confusion when recording dates in your family tree, it is best to use the month written in full or an alphabetic abbreviation such as Nov for November, rather than using a month number, such as 11 for November).

2. Failure to recognise and convert Julian dates to Gregorian dates

The Gregorian calendar was first introduced by Pope Gregory in Venice in 1582, and replaced the old Julian calendar. The new calendar altered the way that leap days were calculated and added days, so that the length of an average calendar year better matched the length of a solar year, ensuring that Christian religious festivals were celebrated at the correct time of year.

Different countries moved from the Julian calendar to the Gregorian calendar on different dates. France, Spain and Portugal were early adopters in 1582. Most Protestant countries, however, did not accept the Gregorian calendar until the 18th century, such as Great Britain and its colonies in 1752. Some countries did not adopt the new calendar until the early 20th-century, such as Russia in 1918.

“The marriage took place today, July 7, 1903, of Szmelka Kurland… to Sura Rozenbaum …” This is an extract from a marriage record from the town of Miechow, Poland. According to the local calendar the marriage took place on 7 July 1903. However to interpret this properly, we need to understand that in 1903, Miechow was located in the Kielce province of the Russian Empire, and that Russia was still using the Julian calendar. There are several online Julian to Gregorian date converters. This gives the correct corresponding Gregorian date, used by most other countries in 1903, as 20 July 1903 – or 13 days later.

Do you have ancestors in your tree from various locations and time periods? If so, we recommend you enter the event’s main date according to the Gregorian date for consistency, but also to note the original Julian date.

Another common mistake in understanding the Julian calendar is to assume that the first day of the new year – the day on which year changed – was 1 January. Often it was not. For example, in England and its colonies, between 1155 and 1751, each new year started on 25 March! So, in England, the day after 31 December 1749 was 1 January 1749. And the day after 24 March 1749 was 25 March 1750.

This is illustrated by a list of 1749 burials for Norwich, Norfolk, England as recorded in the Norfolk Bishop’s Transcript registers (available on MyHeritage), see above. The burial entries are sequential and correctly show the burial of Anne, the wife of James Goodbody on 27 November 1749, before the burial of Bridget Howman, buried on 12 January 1749.

Dates between 1 January and 24 March were sometimes written as “double dates” – such as February 17, 1745/6 (where 1745 was the Julian Year). Some family historians prefer to write Julian dates in this double date format.

3. Misunderstanding dates from other special/religious calendars

Throughout the ages, and in different locations, many different calendars, in addition to Julian and Gregorian, have been used.

For example, from late 1793 until 1805 the French Republican/Revolutionary calendar was used in France. One objective was to remove religious influences from the calendar. Each week had 10 days with the 10th day of each week being a day of rest and festivity replacing the Sabbath. Each month had three weeks and there were 12 months in a year. In addition, at the end of each year, around mid-September, five or six extra days of festivities were added to make the year 365 or 366 days long. Again context is important; if you have a French document from this time, it is then important to convert the French Republican date to a Gregorian date to aid understanding. Fortunately, there are several calendar converters, including one within MyHeritage’s Family Tree Builder software.

Similarly, some religions have their own calendars. The Islamic or Muslim calendar is a lunar-based calendar of 12 months of 29 or 30 days, for a total of 354 days. Unlike many other calendars, there is no attempt to keep the festivals in the same season each year. Consequently, each year, Muslim festivals and dates occur some 11 days earlier in the Gregorian calendar than in the previous year.

The Hebrew or Jewish calendar is a lunisolar calendar. The months are lunar-based and have 29 or 30 days each. However, unlike the Muslim calendar, every two or three years a leap month is inserted into the year so that Jewish festivals continue to fall during the same season in each year. It is important to remember that each Hebrew calendar day starts at nightfall and lasts approximately 24 hours, so if an event starts during an evening then it is recorded with the Hebrew date of the following day.


The Hebrew inscription on the headstone above reads “Here lies the honorable Iser, son of (Mr.) Tzemach, died on the first day of the week [Sunday] 13 Sivan in the year [5]684. The Hebrew date can then be converted into the Gregorian date of 15 June 1924 using the conversion aid in Family Tree Builder, or date converters available online.

4. Confusing an event’s date and the date when it was registered or recorded

Events are often officially registered or recorded after the date when they actually occurred.  In England, for example,  there is a six week period following a birth during which time the birth should be registered. Also the indexes for English births for 1837-1983 were organised by quarter year, see below.

The birth entry for HARRIS, Kevin M., which appears in the register for January, February and March 1954, could well refer to a birth that occurred during the last six weeks of the year 1953 (and possibly even earlier if registration was not undertaken within the normal six-week period). It would be a mistake to conclude from this 1954 entry that the birth took place in 1954. One way to be sure would be to order a full birth certificate which would record the actual date of the birth.

5. Assuming that the dates in a document were recorded accurately

Documents may contain date errors for a variety of reasons. There can be errors if the respondent (who provided the information) never knew the accurate event date or their memory of the actual date has faded. In such a situation, the respondent may have guessed or approximated the date, especially if they were not present at the event. This happens frequently on death certificates when the deceased’s date of birth is required and the informant may know only the approximate age.

Also, when providing information, there could be reasons why the date might be deliberately falsified. Perhaps a person wanted to marry against the wishes of their parents but had not yet reached the required age, then they might have falsified their age or date of birth.

Errors also frequently occurred due to mis-hearing or mis-understanding by either the respondent or the person recording the information.

Consequently, a document’s dates and ages should ideally be cross-checked against other independent sources for the same data, for both reasonableness and consistency.


Always look at the original context of the event and, ideally, at the document in which it was first recorded to avoid errors in understanding as to when the event actually occurred.

For additional confirmation, especially where there may be doubt over its accuracy or interpretation, always try to find one or more additional independent documents relating to the same event.

When recording dates in your own records – if you standardize a date to a modern day calendar/standard – always note any change you have made with details of how the date was originally written and, where possible, include an image of the original text. This can be easily achieved when entering a fact into your MyHeritage family tree.

Have you come across any obstacles deciphering dates? Do you have other tips or mistakes to avoid? Let us know in the comments below.


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  • Anne Smith

    January 13, 2015

    Thank you. very informative

  • John Wood

    January 13, 2015

    Thank you, Emma. This was very helpful to my understanding.

    I’ve only had a vague understanding of issues with “dates”. I have pretty much always just shrugged my shoulders and plowed forward, not knowing; “what made this date a double-date year?”. I appreciate the process of interpreting the dates better now.

  • Mike

    January 13, 2015

    Very helpful. Thanks!
    13-1-15 😉

  • Christine Cleal

    January 14, 2015

    Thanks! Minefields ahead as I check!

  • Esther

    January 14, 2015

    Something to be aware of with Irish ‘birth’ dates.

    My mother b.1920 in rural Ireland, had no need for a birth cert until the late 1990s when rules were changed so that anyone flying into the RoI from the UK had to have a form of official identification.

    At this point a load of elderly people found themselves having to apply for their passport, and before they could get that, they needed their birth cert.

    My Ma’s birthday had always been celebrated on 19th. Her birth cert, which she’d never seen until then, said she was born on 17th. My cousin’s mother found she was three weeks older than she thought. These aren’t the only cases I’ve heard of where the supposed birthday is not what’s on the birth cert.

    I believe the anomaly arose because most people used their baptismal certificate as ID – the date of baptism became their birthday till proven otherwise. The baptismal cert was given out immediately by the priest. My grandparents didn’t even register the birth of several of their children till the mid-1920s when they did them in bulk after the legislation of the time became more forceful. So getting a birth certificate wasn’t a priority, and it cost money to get one, whereas for Catholics, the baptismal cert was their key to Heaven and work and getting schooling and all sorts of things that the trivialities of civil registration couldn’t do.

  • Rick

    January 14, 2015

    An exzcellent article, thank you very much, I have often worried about this!

  • Anne

    January 14, 2015

    Thankyou – some very interesting points in your article. I understand better now about interpretation of dates. Also too in Scotland about 1840’s there was a Church Disruption which meant some could not record birth, death or marriage in church because of break a ways.

  • Linda

    January 14, 2015

    Ester is very correct. Many would register a birth, have a mother child the following year, the first child pass away and not register the second child. This was due to cost.

  • David

    January 18, 2015

    Is there a way to record a date using a calendar other than the modern Gegorian calendar? That way, a date converted to the calendar of today might be represented by the original recording calendar also.

    The French revolution calendar is significant too. Was it dropped when Napoleon began conquering other areas of Europe? Were there areas of Europe and associated colonies world-wide subject to the French revolution calendar?

  • Phyllis

    January 18, 2015

    Some family members have lied about their year of birth to “appear” younger. I have tried to use the birth year given by the parents on census records when the family member being researched was 10 yrs or under.

  • Sue Fox

    January 19, 2015

    Thank you so much for the information you have provided. This can be very usefull to me now with some dates recorded in the 1600/1700’s in the colonies and in England. There has been some confusion with some and now I can see why.

  • Dennis Moules

    January 23, 2015

    Very interesting report I have come across several of these problems over the years and had a converter down loaded to the pc.
    This helps when noting that the birth/marriage can differ from one family tree to the next family tree you encounter.
    Also some census record’s can give you conflicting dates to the information you may have recorded.
    I find that recording the separate dates in my paper record’s till such time as the correct information can be added to the family tree helps.
    As we delve deeper into our family history the information we find is only as accurate as the recorder/person collecting or giving the said information at that point in time.
    Just one more learning curve on our journey to tracking down our ancestors.

  • Jo-Ann Wilson

    January 23, 2015

    Thanks for giving this informative information. This clears up a lot of issues that I could oit understand before.

  • Bob Capp

    January 23, 2015

    I can relate to many of these comments- Being raised Ukrainian Orthodox we were quite familiar with the Julian/Gregorian differences.
    Also, my dad said he celebrated his birthday 12 Dec until 1939 when as part of applying for US Citizenship he found it was recorded as 09 Dec. Grandma said it was around St. Nicholas Day but the midwife reported it differently.
    Then there’s grandma who lied about her age when she applied for insurance, so in her obit Dad used the age she gave them not her actual age.
    Isn’t it fun?

  • Pat Masters

    January 23, 2015

    Thanks for this info. It clears up dates for me. I would see different birth dates for the same ancester, was confused, it really helps. Now I know it could be the same person.

  • Eric Rice

    January 23, 2015

    As Ester-Jan 14 2015 indicated I find a lot of the Genealogy sites show birth dates when they are baptismal or christening dates which could occur several months or even years after the actual birth date. I recently found a relatives christening record 7 years after her birth. I had also been indicating a birth between Jan to March and didn’t realize it could be the previous year. Thank you for that. I will treat that like I do Christening or Baptismal records and indicate “circa” in front of the year.

  • D Robison

    January 23, 2015

    I have noticed that there seem to be more birthdays and deaths on Christmas (25 Dec) than probability would lead one to expect. Is this true, and, if so, is there a reason for it?

  • Coral

    January 23, 2015

    Very, very helpful. I hadn’t actually registered this as being relevant particularly in Gt Britain when Gregorian calendar accepted in 1752, prior to which I have dates for some relatives. I was certainly not aware of the new year starting from 25th March either. I also have experience of a Nan & Granddad changing their birth dates on their marriage certificate to disguise their ages.

  • Elizabeth Mccaffrey

    January 23, 2015

    Thank you very much for the information I did wonder that how reliable are the sources as I did fined it hard to match some dates to what I already have. Some where way out to the other I can date back the family to 1113 but got difficulty to fined the closer family links us they lived all over the places.

  • Richard H L

    January 23, 2015

    The case that bests describes the problem is the Irish event known as the Battle of the Boyne. Reading contemporary accounts of the battle refer to July 1 1690.
    The event is now celebrated on the July 12th each year.

  • J


    January 23, 2015

    Interesting factoid about Jewish Birth/Death Years. My paternal
    side (way back) was Jewish. Nice that some would mention days b/d’s fell on. Will have to convert Jewish dates to our calendar.

  • Elizabeth

    January 24, 2015

    Thank you for the article especially the English records. My father and his twin sister were born in early December but were not registered until the first quarter of the following year. Hence their birth date is wrong in some records

  • Jane Fallon (shaw)

    January 24, 2015

    I am very grateful for this info. I am also grateful that my relatives didn’t go anywhere! It must have been an uneventful existence, but it has made my research easy. Thank you for your guidance.

  • Candy S

    January 24, 2015

    Another mistake I have found is in interpreting early Quaker records. “First month” prior to 1752 was March, not January, yet I frequently encounter this error when looking at records posted according to the Gregorian calendar of today, particularly on Find a Grave.

  • Liz

    January 24, 2015

    One thing not mentioned is that when the Gregorian calendar replaced the Julian calendar, the days were re-aligned – ie there were 10 or 11 days missing, I think it occurred as :
    Day one 1st October day two 11th October

  • Lily

    January 25, 2015

    When my father was interviewing elderly relatives in the 1970’s he found that many of them were vague about the exact birthdates of some of the children due to the stigma surrounding being born outside of wedlock when they were growing up, so that’s something else to consider.

  • Patti Gottschall

    January 26, 2015

    Very good article. One question: When a newspaper says “So and So died on the 12th inst.” What does “inst.” mean?

  • Joanne H

    January 26, 2015

    Thank you for the information. In New Zealand the Maori people were not recorded until it was made law in about 1950 hence trying to find correct birthdates were difficult despite baptismal records being available and as others say were often years after birth. Also when missionaries arrived they wrote dates for births as they saw it and even changed (anglosized) maori names to English. I too believe that the History of the period needs to be taken into account. Thank you for the enlightment.

  • Michael S

    January 26, 2015

    Coral (23rd Jan.): The supputation of 25th March for the start of the year did not apply all across Gt Britain in 1752: to England, yes, but Scotland already started the year on the Feast of the Circumcision (1st Jan.).

  • Lou Berthelot

    January 26, 2015

    Avoid confusion by using the Système International (SI) which gives dates from largest to smallest number e.g.
    2015-01-26-16:42:33 or if you want at 1642 hours and 33 seconds on January 26th in the year 2015.

  • David Shaw

    January 26, 2015

    My gt. gt. grandmother was born in Scotland. The 1841 Census says she was 50. However, in that census only, the age of an adult was usually rounded down to the nearest number ‘5’. So she may have been aged 50-54.

    Not very helpful when I know she was married on 27 Nov 1807, and her husband had died before the census. They were married in a secession church and no records were available for their dates of birth. Apparently she was born about 1786/7. Her husband? Perhaps between 1770 and 1790: perhaps in the records, probably not. Everything is fine between then and now!

  • netace

    January 27, 2015

    Some of our departed seem to have been determined to mess up any government records, censuses in particular. Net result, each time a census was taken, a different age was given – no consistency at all, different place of birth, yrs married, etc.

    In the Northern Ireland area of Carnmoney in the early 1800’s, William Fee McKinney was a literal god-send since he recorded all of the Birth, Marriage and Death info that occurred in the areas that he travelled. Unfortunately, many areas did not have a person so conscientious as McKinney.

  • Malka Marilyn Greenberg, Indig, Shavit Israel

    January 27, 2015

    Thanks for this excellent report. I have been making Family Trees for my parents’ parents and as they were born in Eastern Europe, cannot get any info earlier than approx 1880. My father’s parents died in 1942 and 1948 in Manchester, UK. My mother’s parents died in 1960 and 1968 Manchester. Very little info again re ancestors, as many died in the Holocaust in Poland and Roumania, Moldova, Town of Vaslui.

  • Margret Kaul

    February 3, 2015

    Even the good old US Census can lead you astray! When an age is given in 1900 as “9”; in 1910 as “18” or “19”; and in 1920 as “28” when was he born? The answer could be anywhere from June 1890 to Dec 1891 due to the different dates of the “census year.” 1900 is as of 1 Jun 1900; 1910 is as of 30 April 1910; 1920 is as of 1 January 1920. I try to find the 1920 Census as no matter the month the “age” can be subtracted from the year “1919”

  • Don Roddy

    February 4, 2015

    A peculiar case: My grandmother’s birth certificate shows she was born in 1881, and that is the year she claimed for her birth. Her mother also listed my grandmother’s birth occurring in 1881 in a deposition made in 1921. Imagine my surprise when I found my grandmother listed as a four month old infant in the 1880 census! (all other censuses reflect an age consistent with her birth being in1881).
    It turns out that my grandmother’s birth certificate was issued at the direction of an Oregon court (she was born in Nebraska) in the 1940’s based on information provided by my grandmother and her younger sister. It appears that my great grandmother had a memory lapse early on and had always thought her daughter was born one year later than she actually was.

  • Don Dacon

    July 5, 2020

    my date question deals mostly with the census. Jonathan O Dacon is my research person (who is my 2nd great father).
    1850 census shows his age as 12 leaving him to be born in 1838.
    1880 census shows his age as 38 leaving him to be born in 1842.
    my family bible records show him to be born in 1840.
    If the 1850 census is correct I can tie him to David Dacon as his father. (which i would love to do)
    However DNA says I may be from a Hubbard line. I am very confused.
    Can you please give me some guidance? Thanks..Don Dacon