Julian Calendar vs Gregorian Calendar – What You Should Know!

By | October 20, 2017

Exploring your family history will eventually throw up some issues for you when recording dates. This Julian calendar vs Gregorian calendar post will help you make sense of what dating system your ancestors used. I will also show you how to convert these old dates and explore best practices that you should adopt.

Julian Calendar vs Gregorian Calendar

Which date should you record for your genealogy research?

When it comes to tracing our family tree and gathering information about our ancestors dates are pretty much an important factor. We use them to record the special moments in the lives of our relatives.

For example, we will record when they were born, when they married and when they died. We may also record other important events during their lifetime as well.

There is no issue recording dates when it comes to researching our most recent ancestors. It is only when we are tracing our family tree prior to 1752 that we need to consider what date we shall enter.

Do we go with the Julian calendar that has existed since 46BC or the revised Gregorian calendar that came into effect for Britain and the United States in 1752?

Confused? Well I will explain the difference between the both of these calendars and help you to understand them a little better.

Credit:   Ancestry

Julian Calendar

The Romans adopted ancient Babylonian calendars as well as dating systems from other cultures. It was Julius Caesar in 46BC who wanted to adopt a new calendar which could be used to calculate when to accurately sow and harvest their crops. And so from that point on the western world mostly used this form of keeping track of events.

Julian Calendar

How many seconds, minutes, hours, days and weeks?

From the Mesopotamians the Romans would divide an hour into 60 minutes, and a minute into 60 seconds. Ancient Egyptian calendars would help to break up a day into 24 hours, while the Jewish calendar would see 7 days make up a week.

As the Roman Empire was mostly an agricultural society at the time of Julius Caesar a new system was therefore needed for them to survive. The seasons of the year would obviously play an important part. And so the year was broken into 12 months and would include 365 days. This division was based on the solar cycle.

The Romans knew however that they would need to add an extra day every 4 years. This is because the average year was actually 365.25 days and not exactly 365 days.

As the Roman Empire spread across Europe this calendar was adopted by many western world countries. After the Roman Empire ceased to be the Roman Catholic Church would use and maintain this calendar for many years to come.

Gregorian Calendar

For many centuries the Roman Catholic Church relied on the Julian calendar to mark when Easter began, (and from then on other religion events). This date is calculated by knowing when the spring equinox is, which is exactly when there are 12 hours of daylight and 12 hours of darkness during the day.

Knowing when the spring equinox was important as Easter is celebrated on the first Sunday following the full moon after the spring equinox. However, during the 1500s the Roman Catholic Church would realize that there was something wrong when the spring equinox was being incorrectly calculated.

Gregorian Calendar

But why was spring equinox marked incorrectly?

Well it all comes down to how many days there really, really is during the year. The Julian calendar worked on the impression that there were 365.25 days in a year. But this is not actually the case.

There really, really is 365.242199 days in a year!

You may not think that that makes much difference to calculating how long a year is BUT after every 129 years the Julian calendar would be 1 day out of synchronization with the solar year. Something therefore needed to be done. But what?

Pope Gregory XIII and the change that was needed!

Several commissions tried to find a solution to this problem. As I have already mentioned if Easter was incorrectly celebrated then other religious occasions would also be wrong. It was therefore in 1582 that Pope Gregory XIII would issue a papal bull that would revise the calendar that they used.

Incidentally the church did know of this problem well before the general public realized that something was wrong. It was only then did the church knew that they needed to act.

The Romans knew that they would have to add an additional day every 4 years. But the Catholic Church adopted the practice whereby an extra day was NOT added if the year was divisible by 100. Although there was the exception that if the year was divisible by 400 then a day would not be added.

Hope I’m not confusing you!

This change therefore meant that 1700, 1800 and 1900 were not leap years, but 1600 and 2000 were however leap years. Why not check the 1900 calendar and see for yourself that there was no February 29th for that year.

Over time this minor difference to the number of actual days in a year resulted the Julian calendar being 10 days out of synch. The new Gregorian calendar would therefore need to omit these days from their improved calendar.

Adopting the change!

So Thursday October 4th 1582 in the old Julian calendar was immediately followed by Friday October 15th 1582 in the new Gregorian calendar.

Italy, Spain, Portugal and Poland would immediately adopt this new calendar, while Belgium, France and the Netherlands would accept this change in December of that year.

Other European countries were however slow to accept this change. Denmark, Sweden, England and its colonies were distrustful of the Roman Catholic Church.

It was not until 1752 that England its colonies would follow the rest of Europe and adopt the Gregorian calendar. By this time the number of days out of sync had accumulated to 11. This therefore meant that they would have to drop these 11 days in order to ‘catch up‘ with the rest of Europe.

So for England and its colonies Wednesday September 2nd 1752 was followed by Thursday 14th September 1752. And so the dates in between do not officially exist, (but were still used by some – more on this later).

Making Sense of the Gregorian Calendar!

Hopefully I haven’t confused you too much thus far. I have tried to keep it as simple as possible. I’m sorry to say though that there is more that you need to consider for when you are researching your ancestors and are trying to keep track of dates.

Making Sense of the Gregorian Calendar

Problems adopting the new Gregorian calendar!

As I have mentioned some European countries did adopt the new Gregorian calendar immediately but others lacked behind. So you need to consider this when researching your ancestors from those countries.

Also, even though the calendars were changed at various dates for these European countries the general public did not accept it and so stuck with the old dates. They felt that there lives were being shortened by this change.

So when you are researching old family records and documents you will need to be mindful of this.

There is some relief though as you may either see OS, or NS written in these records and documents. This key will help you as it will tell you that either the date is in the Old Style (Julian calendar), or the New Style (Gregorian calendar).

When is the New Year?

There is though one more issue that you will need to be mindful of. And that is our ancestors did not actually celebrate the New Year on January 1st.

So when did they celebrate New Year then I hear you ask?

Well, depending on where you ancestors lived it may have been on December 25th (winter solstice), or March 24th or 25th (spring equinox). Britain and its colonies used the spring equinox to mark the start of the New Year. So if you are researching your family tree before 1753 you will need to be mindful of this.

It is a bit confusing so here is an example:

If your ancestor was born sometime between January 1st and March 25th of 1751 then the actual new date would instead be 1752.

The Julian Date Calculator

Now that you know of these date issues when it comes to researching your family tree you will need to yourself adopt a method of recording dates. You could enter the Julian date that you find for your ancestor. Or you could enter the correct Gregorian date.

Which ever one you use you will need to highlight this. And obviously stick with it, otherwise you will end up all confused.

Julian Date Calendar

Converting Julian date to Gregorian date

For example, say that your ancestor was born on January 1st 1751 and this is the recorded Julian date of birth. You therefore want to convert it to the Gregorian date. This can be done by a 2 step process.

  1. Add 11 days to January 1st so no it becomes January 12th
  2. Then add 1 year so that it is now 1752, (only necessary if the Julian date is between January 1st – March 25th)

As I have just pointed out you need to stick with one date or the other. You could write the above example as January 1st 1751/52, but this is not technically a date.

This therefore could lead to confusion so please do not adopt this practice!

Thank You and Please Leave A Comment

I hope you enjoyed this post explaining the difference between the Julian calendar vs Gregorian calendar. And I hope that it has helped you to understand dates a little more when it comes to genealogy research.

If you think that this post will be helpful to others then please share it with friends and family using the social media buttons below.

If you have any questions or comments then please leave a comment below. I would love to hear from you.

10 thoughts on “Julian Calendar vs Gregorian Calendar – What You Should Know!

  1. Kayla

    Thank you for this information! I had no idea there could be such a disparity between the dates and days that are being used. I have always wanted to start doing some research into my family history and try to go as far back as I can so long as there are some sort of records. I know I will eventually run into a major snag but I think this article will give me a better chance at learning about my history. Thanks!

    1. Owain Post author

      Glad to be of help Kayla. When you do get this far back with your family history research this guide will sure be of use to you. Please check out the rest of the site for more helpful guides and tips.

  2. Alice

    Although you got my head really spinning, you did a pretty awesome job in explaining the difference between the Julian and the Gregorian calendars. Oh, I have to admit, I had to read your article a couple of times to make sure I understand what you’re saying.

    Anyway, you said that in order to come up with the correct date using the Gregorian calendar, we need to add 11 days to the date as recorded in the Julian calendar and add 1 year, is this applicable to any date?

    My cousin has been working really hard to research our family tree and I thought I could lend her a helping hand by looking into the problem with regards to the dates.

    And since we’re talking about calendars, how is the Julian calendar different from or similar with the Hebrew calendar? I heard that the Jews have their own calendar too and that’s why they don’t celebrate New Year on January 1st.

    1. Owain Post author

      Hi there Alice,

      It does take a while for it to sink in. There are a few considerations that you need to be mindful of. You only need to apply this correction to 1751 and before.

      Both the Julian and Hebrew calendars are similar in that they are used to tell when to celebrate religious events. Although the Julian calendar did become out of sync.

      You might be interested in a Gregorian and Hebrew date converter that I have found at Hebcal.

  3. Carole

    WOW very interesting post. I never knew how to convert the Julian date to Gregorian date. Thanks for this great information!

    1. Owain Post author

      You’re quite welcome Carole. It did take a while to properly research this subject and to try and make sense of it all. Hopefully I haven’t confused you or anyone else for that matter.

  4. Suzanne

    Hi Owain,
    Thank you for so nicely explaining the differences between these two calendars. Fascinating information!! I can certainly see why some people would have difficulty tracing their family tree. If and when I know of any friends doing a search that far back, I will definitely send them to your post.
    Cheers, Suzanne

    1. Owain Post author

      Researching family history can be quite tricky at the best of times. So changes with the date can really get your head in a spin. I am glad that I could be of help. All the best with your family history research.

  5. Jeremy

    Hey Owain that was very insightful, I had no clue and hadn’t even considered to incorporate the different calendar dates in order to find my ancestry.

    Will definitely know to keep those two calendars in mind if I undertake some research.

    Thanks Owain!

    1. Owain Post author

      You’re quite welcome Jeremy. If you do go prior to 1750s then you will need to keep these calendars in mind. Glad to help.


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