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This page is about how to enter dates in forms, and how this site deals with the different calendars used in the past.

For quick reference, values for most date fields in forms should be entered like this:


JL stands for the Julian calendar and GR for the Gregorian calendar (see below for more explanation of this).

For example, this is the date of the battle of Edgehill in each calendar:

  • 1642-10-23 JL
  • 1642-11-02 GR

The Julian and Gregorian calendars

The Julian calendar was used in most of Europe from ancient Rome until the 16th century. It got increasingly out of sync with the solar year because it had too many leap years. In 1582, the Catholic Church introduced the Gregorian calendar, which corrected the error. The new calendar was soon adopted by Catholic countries, and by some parts of the Netherlands, even though they were protestant and had already declared independence from Spain. Many other protestant and Eastern Orthodox countries only changed much later.

Great Britain and Ireland changed from the Julian to the Gregorian calendar in 1752, so documents from the British Civil Wars usually have Julian dates.

In the 17th century, the difference between the two calendars was 10 days. For example, 10 October Julian was 20 October Gregorian. The difference increased to 11 days in 1700 and is now 13 days. But note that the transition from Julian to Gregorian only changed the number of the day, not the day of the week, which is the same in both calendars. For example, the battle of Edgehill was fought on 23 October 1642 Julian, which was 2 November Gregorian, but it was a Sunday in both calendars.

Letters sent from a place that used one calendar to a place that used the other sometimes have double dates such as 10/20 October. This isn't always the case but you can tell which calendar was used if it also gives the day of the week. For example, Sunday 23 October 1642 must be Julian because 23 October 1642 Gregorian was a Thursday.

Ian's English Calendar can calculate the day of the week. You can also use the date tag on this site to automatically convert between calendars and find the day of the week (see below for more details).

For more information about calendars, see these Wikipedia pages:

WARNING: Wikidata can store both Julian and Gregorian dates, but data has often been entered with the wrong calendar, so don't trust it.

The Julian calendar is sometimes known as Old Style and the Gregorian calendar is sometimes known as New Style, but these terms can also refer to the start of the new year, which is a different problem. See the next section below.

The Old Style and New Style new years

New Style new year means starting the official year on 1 January (New Style can also refer to the Gregorian calendar, which is a different thing: see section above).

In Great Britain and Ireland, the Old Style new year was on the 25 March after 1 January (other Old Style new years have been used in other places and times, but these don't affect this project).

  • Scotland changed to the New Style new year in 1600, but kept the Julian calendar until 1752
  • England and Ireland changed to the New Style new year in 1752, at the same time as changing to the Gregorian calendar

Before these dates, documents dated from 1 January to 24 March can be ambiguous. Although the official new year started on 25 March, many people treated 1 January as the new year. For example, George Thomason, who collected the Thomason tracts, often wrote the Old Style year onto pamphlets that were printed with New Style years.

Modern printed editions of manuscripts often disambiguate dates by giving a double year. For example, 1 March 1644/5 means 1644 Old Style (year starting 25 March) and 1645 New Style (year starting 1 January).

Secondary sources often treat the year as starting on 1 January, and have a note at the start to say which calendar and new year style they are using.

This site follows the practice of most secondary sources: we treat the year as starting on 1 January, but otherwise use the calendar used in the original source. This means that if documents were created in England or Ireland between 1 January and 24 March in any year before 1752, you will need to convert the year to New Style but you don't need to convert between Julian and Gregorian dates.

WARNING: some online resources don't take the Old Style new year into account. Parish registers at FamilySearch can be misdated.

Entering dates in form fields

Semantic properties that have the type date are entered through form fields. If the form field doesn't yet have a value, it will display the text YYYY-MM-DD JL as a hint. This text will disappear when you start typing in the field.

Values for date fields in forms should be entered like this:


That means a 4 digit year number, a 2 digit month number, and a 2 digit day number followed by JL for the Julian calendar or GR for the Gregorian calendar (see above for more explanation of this). There must be hyphens between the numbers, and a single space before the calendar code.

For example, this is the date of the battle of Edgehill in each calendar:

  • 1642-10-23 JL
  • 1642-11-02 GR

Always specify which calendar you are using to avoid ambiguity (Semantic MediaWiki will make assumptions about the most appropriate calendar if you don't specify, but it's best practice to always be explicit).

You can enter a date in either calendar. It will automatically be displayed on the page with both Julian and Gregorian dates, and the day of the week.

It's usually best to use the same calendar as the documents that you are entering data about or using as evidence. If the document gives a double date with both the Julian and Gregorian day, you can use either but do not enter both at the same time as it won't work.

All dates entered in these form fields, whether Julian or Gregorian, should use 1 January as the start of the new year. You might need to modernize the year if it's from 1 January to 24 March. Some sources, especially modern printed editions of manuscript sources, might give a double year, such as 1644/5. In these cases, always use the second year given.

Uncertain dates

Date properties can store incomplete dates. You can leave out the day or month if they can't be known. For example:

  • 1642 JL
  • 1642-10 JL

You should still specify the calendar in the source that the date comes from, as the discrepancy between the Julian and Gregorian calendars means that the months and years don't quite match.

You must always specify a year. Dates where the month and day are known but not the year can't be stored as semantic data. In these cases, just leave the date field blank but mention the incomplete date in free text somewhere else on the page.

Multiple dates

Most date fields can only take one date. The only exception is the date of a combat event. This can take multiple dates separated by a semi-colon if the event went on for more than one day.

Years only

The only exceptions to this format are:

  • an author's year of death
  • year of publication of a book or article
  • year of completion of a thesis

These only take a year number with a maximum of 4 digits. This is stored as a number, not a date.

Entering dates in transcripts and free text

You can use the XML tag <date> to mark up a date in transcribed text, or format a date in free text. This tag formats the date displayed on the page but does not store the date as a semantic property. It's similar to the TEI element <date>, but not exactly the same.

The date tag should always have a single attribute called 'when'. The value of this attribute should be a date in the same format as dates in form fields (see above). For example:

<date when="1642-10-23 JL" />

Just like form fields, the date can be entered as Julian or Gregorian, and you must use JL or GR to specify which one. This is different from the TEI date element, where @when must always be a Gregorian date in ISO format. TEI dates can also have many other attributes which we don't use here.

Marking up dates in transcribed text

In transcripts, summaries or extracts of original sources, you can use the date tag to mark up strings that refer to dates.

In this case, you should always have a separate opening and closing tag with contents between them.

The contents of the tag should be the original text. This will be displayed as it is.

The value of @when should be the standardized form of the date. You should usually use the same calendar that the original source uses. If it specifies a double Julian and Gregorian date, such as 10/20 October 1642, you can use either calendar, but do not try to enter both at the same time as it won't work. The standardized date will automatically be displayed in a footnote as a full date in both Julian and Gregorian calendars, with the day of the week.

For example, if you type this:

<date when="1642-10-23 JL">when Edgehill fight was</date>

it will be displayed as:

when Edgehill fight was[1]

If the day or month isn't known, @when can take incomplete dates in the same way as form fields. In this case, the footnote will only display a partial date in the same calendar that you specified, not a double date, because months and years without days can't easily be converted from one calendar to another.

If the year isn't known, or the date referred to can't be worked out at all, then it's not worth marking up the text as a date.

In 17th-century diaries and letters, it can be common for the day of the week and the number of the day to go out of sync. In these cases, the day of the week can sometimes be more reliable than the date. It's sometimes possible to work out the correct dates by following the sequence to where it went wrong.

Formatting dates in free text

In free text other than transcripts of original sources, you can use the <date> tag as an easy way to format a date with the correct day of the week and equivalent Julian and Gregorian dates.

In this case, date should be a single self-closing tag with no contents. @when should usually be a full date including year, month, day and calendar. If you don't specify a day, it will only display the partial date in the calendar that you specified, not a double date.

When used like this, the tag does not create a footnote. The formatted date will be displayed inline where you type the tag.

If you type:

<date when="1642-10-23 JL" />

it will be displayed as:

Sunday 23 October 1642 JL / 2 November 1642 GR

Displaying dates in query results

Query results display one date per column. By default, each date will be displayed in the same calendar that was specified when the date was entered. If you're writing your own query, you can force it to display dates in a certain calendar by appending:

  • #JL for the Julian calendar
  • #GR for the Gregorian calendar

To display double dates, you will need to add a separate column for each calendar and specify the calendar using the codes above.

Note that in query results, Julian dates will be indicated with the suffix JL in superscript, but Gregorian dates won't have any suffix, unlike when they are displayed on the page they were entered on.

You can also change the formatting of dates in query results in other ways, and generate sort keys to sort query results into date order. For more details, see Semantic MediaWiki help page for dates.

Note that if your query uses a date format that displays the day of the week, Julian dates will have the wrong day because of a bug in Semantic MediaWiki. This does not affect the way that dates are displayed on the pages where they were entered, where the day of the week shown will be correct for both the Julian and Gregorian calendars.

  1. Sunday 23 October 1642 JL / 2 November 1642 GR