Julian-Gregorian-Dee Date Calculator allows: • conversion of dates in four calendars: Julian, Gregorian, Dee and Dee-Cecil • user-preferred year designators: AD, BC, CE and BCE • user-preferred date input format: popular or ISO 8601 with astronomical year numbering • identification of calendar by combination of month,day,year with a day-of-week • addition/subtraction of days, weeks, months and years to a given input date • calculation of dates and times of vernal equinoxes for years 1 through 4000 CE in the Gregorian and Dee-(Cecil) calendars.
Astronomical Year Numbering In the Julian and Gregorian Calendars years prior to the year 1 are numbered by counting backwards from that year and adding a suffix (a.k.a. a year designator) to distinguish years prior to the year 1 from years following the year 1, e.g., 1 B.C., 2 B.C., and so on. Many astronomers and calendrical scientists prefer to use a system of numbering years prior to the year 1 using zero and negative numbers: ... -2, -1, 0, 1, ...
Addition and Subtraction of Days etc. After an input date has been completely specified you can specify a number of years, months, weeks and days to be added or subtracted from it. Clicking on the Calculate button then displays the result of adding or subtracting the specified periods to or from the input date. Years are added/subtracted first, then months, then weeks, then days. (This function is not enabled in the trial version; to use it the software must first be activated.) If the result would be an invalid date in a particular calendar (e.g., adding 1 month to 2011-01-30 CE) then that date is decremented by one day until a valid date is reached (in this example, 2011-02-28 CE).
If the input date is fully specifed then the Date Sequence button is enabled. This allows display of a sequence of dates, beginning with the input date, for up to 800 days, for the Common Era Calendar and the corresponding dates in the Dee, or Dee-Cecil, Calendar. This is mainly so as to observe how the CE dates are correlated with the D and DC dates.
An arrow indicates where the CE and Dee(-Cecil) dates begin or cease to accord with each other (or in other words, when they get in or out of sync). In the example above, 2016-03-01 DC follows 2016-02-28 DC because 2016 is not a leap year in the Dee-Cecil Calendar.
The Dee Calendar was proposed as a new calendar by the Elizabethan scholar John Dee about 1580, when mean solar time as we know it did not yet exist (it had to await the invention of accurate clocks). Dee calculated midnight as halfway between two noons (according to a sundial). But noon in apparent solar (i.e., sundial) time may differ by up to 16 minutes from noon in mean solar (i.e. clock) time, with the difference varying with the time of year (see The equation of time). At the vernal equinox the difference between apparent solar time and mean solar time is close to 8 minutes. Thus to obtain the times that Dee would have obtained in his observations and calculations it is necessary to select apparent solar time, which results from subtracting 8 minutes from the mean solar time for the given longitude. This produces the a times shown above for the vernal equinox in the Dee(-Cecil) Calendar.