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Some readers may feel there is something wrong with the use of a proleptic calendar. In reply, you should note that every time you write a date later than today's, you employ a proleptic Gregorian calendar. A proleptic calendar is simply a certain way of looking at time. The use of a proleptic calendar can be pernicious when it is not clearly identified. |
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There are several different applications for proleptic calendars. You may want to look at how dates in one calendar change with respect to another calendar. If the two calendars do not exactly overlap in terms of their historical usage, one of them will have to be used proleptically. Often, you will want to use one uniform calendar system when comparing dates over time. This situation occurs in paleoastronomy, for example. |
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The functions presented here are those needed to convert Julian dates to Gregorian dates and vice versa. As usual, the Julian Day number is used as the means for conversion. |
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Note that if the rules of any calendar can be extended in a regular way, then that calendar can be used proleptically. In the SCDTL system, push comes to shove on this issue in the area of date validation. Date validations in SCDTL are strict, in the sense that they look at historical date ranges. Thus, a date of 5 October 1582 will be rejected by the Julian calendar validation routine because it falls outside of the historical date range of that calendar. |
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It should also be noted that the original "Fliegel and van Flandern" algorithms, which have been used for many years to interconvert dates and Julian Day numbers, are proleptic Gregorian routines. This fact has escaped the attention of many programmers and has caused a good deal of confusion in the literature. |
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The American reader, in particular, should be cautioned that the conversion from the Julian calendar to the Gregorian calendar often involves more than the elimination of some days and the adoption of a new leap year rule. The definition of New Year is often involved. In the case of British possessions, the calendar switch was made in A.D. 1752. Prior to the change, the start of the year was counted from 25 March, not 1 January. Thus, dates labeled Old Style, or O.S., use a start-of-year convention that is not the same as the Julian calendar, as it is implemented by SCDTL. Therefore, the conversion of Old Style to New Style dates is not a simple conversion from Julian to Gregorian dates. |
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Likewise, it is important to note that the switch from Julian to Gregorian calendar was not always accomplished in the smooth and uninterrupted manner implied by history texts. In some cases, the change was implemented incorrectly and corrected later. In other cases, the switch was officially made, but ignored in practice until a later date. In still other cases, the switch was made, the Julian calendar was restored for a period, and the switch was made a second time. |
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In general, if an application deals with actual historical dates, the switch for a particular region or country must be studied carefully, and the historical record of the implementation taken into account. Frequently, this process will involve the creation |
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