Here are three Julian number routines I converted from Mr. Lee's 'C' code to PowerBasic PBDLL

and or PBCC.

Calendar Conversions by Scott Lee

original "C" code can be found here:

Home page http://www.ScottLee.net/

The routines are very fast as they use only integer math.

JDN = Julian Date Number

GregorianToJdn () - Convert a Gregorian calendar date to a JDN

JdnToGregorian () - Convert a JDN to a Gregorian calendar date

DayOfWeek () - Convert a JDN to a day-of-week number (0 to 6), 0 = Sunday

Errol Cheverie

Update - 2001, 1, 19

Updated Nov. 25, 2002

Thanks to Don Schullian for pointing a couple of extra features that were not quite

intended (old age tends to make the mind wonder a bit).

On the DayOfWeek () function this was a literal translation from

Scott Lees "C" code, but I think you're right this will never be

less than Zero. Source has been updated.

In the JdnToGregorian () SUB, you are right the three variables here

were wrong, have now corrected this.

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[This message has been edited by Errol Cheverie (edited November 25, 2002).]

and or PBCC.

Calendar Conversions by Scott Lee

original "C" code can be found here:

Home page http://www.ScottLee.net/

The routines are very fast as they use only integer math.

JDN = Julian Date Number

GregorianToJdn () - Convert a Gregorian calendar date to a JDN

JdnToGregorian () - Convert a JDN to a Gregorian calendar date

DayOfWeek () - Convert a JDN to a day-of-week number (0 to 6), 0 = Sunday

Errol Cheverie

Update - 2001, 1, 19

Updated Nov. 25, 2002

Thanks to Don Schullian for pointing a couple of extra features that were not quite

intended (old age tends to make the mind wonder a bit).

On the DayOfWeek () function this was a literal translation from

Scott Lees "C" code, but I think you're right this will never be

less than Zero. Source has been updated.

In the JdnToGregorian () SUB, you are right the three variables here

were wrong, have now corrected this.

Code:

'============================================================================= ' gregor.c,v 2.0 1995/10/24 01:13:06 lees Exp ' Copyright 1993-1995, Scott E. Lee, all rights reserved. ' Permission granted to use, copy, modify, distribute and sell so long as ' the above copyright and this permission statement are retained in all ' copies. THERE IS NO WARRANTY - USE AT YOUR OWN RISK. '============================================================================= '============================================================================= ' ' These are the externally visible components of this file: ' ' SUB JDNToGregorian(BYVAL jdn AS LONG, _ ' Year AS LONG, _ ' Month AS LONG, _ ' Day AS LONG) ' ' Convert a JDN to a Gregorian calendar date. If the input JDN is less ' than 1, the three output values will all be set to zero, otherwise ' Year will be >= -4714 and <> 0; Month will be in the range 1 to 12 ' inclusive; Day will be in the range 1 to 31 inclusive. ' ' FUNCTION GregorianToJdn(BYVAL Year AS LONG, _ ' BYVAL Month AS LONG, _ ' BYVAL Day AS LONG) AS LONG ' ' Convert a Gregorian calendar date to a JDN. Zero is returned when the ' input date is detected as invalid or out of the supported range. The ' return value will be > 0 for all valid, supported dates, but there are ' some invalid dates that will return a positive value. To verify that a ' date is valid, convert it to JDN and then back and compare with the ' original. ' ' VALID RANGE ' ' 4714 B.C. to at least 10000 A.D. ' ' Although this software can handle dates all the way back to 4714 ' B.C., such use may not be meaningful. The Gregorian calendar was ' not instituted until October 15, 1582 (or October 5, 1582 in the ' Julian calendar). Some countries did not accept it until much ' later. For example, Britain converted in 1752, The USSR in 1918 and ' Greece in 1923. Most European countries used the Julian calendar ' prior to the Gregorian. ' ' CALENDAR OVERVIEW ' ' The Gregorian calendar is a modified version of the Julian calendar. ' The only difference being the specification of leap years. The ' Julian calendar specifies that every year that is a multiple of 4 ' will be a leap year. This leads to a year that is 365.25 days long, ' but the current accepted value for the tropical year is 365.242199 ' days. ' ' To correct this error in the length of the year and to bring the ' vernal equinox back to March 21, Pope Gregory XIII issued a papal ' bull declaring that Thursday October 4, 1582 would be followed by ' Friday October 15, 1582 and that centennial years would only be a ' leap year if they were a multiple of 400. This shortened the year ' by 3 days per 400 years, giving a year of 365.2425 days. ' ' Another recently proposed change in the leap year rule is to make ' years that are multiples of 4000 not a leap year, but this has never ' been officially accepted and this rule is not implemented in these ' algorithms. ' ' ALGORITHMS ' ' The calculations are based on three different cycles: a 400 year ' cycle of leap years, a 4 year cycle of leap years and a 5 month ' cycle of month lengths. ' ' The 5 month cycle is used to account for the varying lengths of ' months. You will notice that the lengths alternate between 30 ' and 31 days, except for three anomalies: both July and August ' have 31 days, both December and January have 31, and February ' is less than 30. Starting with March, the lengths are in a ' cycle of 5 months (31, 30, 31, 30, 31): ' ' Mar 31 days \ ' Apr 30 days | ' May 31 days > First cycle ' Jun 30 days | ' Jul 31 days / ' ' Aug 31 days \ ' Sep 30 days | ' Oct 31 days > Second cycle ' Nov 30 days | ' Dec 31 days / ' ' Jan 31 days \ ' Feb 28/9 days | ' > Third cycle (incomplete) ' ' For this reason the calculations (internally) assume that the ' year starts with March 1. ' ' TESTING ' ' This algorithm has been tested from the year 4714 B.C. to 10000 ' A.D. The source code of the verification program is included in ' this package. ' ' REFERENCES ' ' Conversions Between Calendar Date and Julian Day Number by Robert J. ' Tantzen, Communications of the Association for Computing Machinery ' August 1963. (Also published in Collected Algorithms from CACM, ' algorithm number 199). '============================================================================= $DIM ALL %JDN_OFFSET = 32045& %DAYS_PER_5_MONTHS = 153& %DAYS_PER_4_YEARS = 1461& %DAYS_PER_400_YEARS = 146097& DECLARE FUNCTION GregorianToJdn(BYVAL Year AS LONG, _ BYVAL Month AS LONG, _ BYVAL Day AS LONG) AS LONG DECLARE SUB JdnToGregorian(BYVAL Jdn AS LONG, Year AS LONG, _ Month AS LONG, _ Day AS LONG) DECLARE FUNCTION DayOfWeek(BYVAL Jdn AS LONG) AS LONG '============================================================================= ' small sample for PBCC '============================================================================= FUNCTION PBMAIN() AS LONG DIM JDN1 AS LONG DIM Year AS LONG DIM Month AS LONG DIM Day AS LONG JDN1 = GregorianToJdn(2001, 1, 19) PRINT JDN1 ' should be 2451929 PRINT DayOfWeek(JDN1) ' should be 5 CALL JdnToGregorian(JDN1, Year, Month, Day) PRINT Year, Month, Day ' should be 2001, 1, 19 END FUNCTION '============================================================================= ' Convert a JDN to a Gregorian calendar date. If the input JDN is less ' than 1, the three output values will all be set to zero, otherwise ' iYear will be >= -4714 and <> 0; iMonth will be in the range 1 to 12 ' inclusive; iDay will be in the range 1 to 31 inclusive. ' ' VALID RANGE: ' 4714 B.C. to at least 10000 A.D. '============================================================================= SUB JdnToGregorian(BYVAL Jdn AS LONG, _ iYear AS LONG, _ iMonth AS LONG, _ iDay AS LONG) DIM Century AS LOCAL LONG DIM Year AS LOCAL LONG DIM Month AS LOCAL LONG DIM Day AS LOCAL LONG DIM Temp AS LOCAL LONG DIM DayOfYear AS LOCAL LONG ' correction as pointed out by Don Schullian ' had wrong names for iYear, iMonth and iDay IF (Jdn <= 0) THEN iYear = 0 iMonth = 0 iDay = 0 EXIT SUB END IF Temp = (Jdn + %JDN_OFFSET) * 4 - 1 ' Calculate the century (year/100). Century = temp \ %DAYS_PER_400_YEARS ' Calculate the year and day of year (1 <= dayOfYear <= 366). Temp = ((temp MOD %DAYS_PER_400_YEARS) \ 4) * 4 + 3 Year = (century * 100) + (temp \ %DAYS_PER_4_YEARS) DayOfYear = (temp MOD %DAYS_PER_4_YEARS) \ 4 + 1 ' Calculate the month and day of month. Temp = dayOfYear * 5 - 3 Month = temp \ %DAYS_PER_5_MONTHS Day = (temp MOD %DAYS_PER_5_MONTHS) \ 5 + 1 ' Convert to the normal beginning of the year. IF (Month < 10) THEN Month = Month + 3 ELSE INCR Year Month = Month - 9 END IF ' Adjust to the B.C./A.D. type numbering. Year = Year - 4800 IF (Year <= 0) THEN DECR Year iYear = Year iMonth = Month iDay = Day END SUB '============================================================================= ' Convert a Gregorian calendar date to a JDN. Zero is returned when the ' input date is detected as invalid or out of the supported range. The ' return value will be > 0 for all valid, supported dates, but there are ' some invalid dates that will return a positive value. To verify that a ' date is valid, convert it to JDN and then back and compare with the ' original. ' ' VALID RANGE: ' 4714 B.C. to at least 10000 A.D. '============================================================================= FUNCTION GregorianToJdn(BYVAL iYear AS LONG, _ BYVAL iMonth AS LONG, _ BYVAL iDay AS LONG) AS LONG DIM Year AS LOCAL LONG DIM Month AS LOCAL LONG ' check for invalid dates IF (iYear = 0 OR iYear < -4714 OR _ iMonth <= 0 OR iMonth > 12 OR _ iDay <= 0 OR iDay > 31) THEN FUNCTION = 0 EXIT FUNCTION END IF ' check for dates before jDN 1 (Nov 25, 4714 B.C.) IF (iYear = -4714) THEN IF (iMonth < 11) THEN FUNCTION = 0 EXIT FUNCTION END IF IF (iMonth = 11 AND iDay < 25) THEN FUNCTION = 0 EXIT FUNCTION END IF END IF ' Make year always a positive number. IF (iYear < 0) THEN Year = iYear + 4801 ELSE Year = iYear + 4800 END IF ' Adjust the start of the year. IF (iMonth > 2) THEN Month = iMonth - 3 ELSE Month = iMonth + 9 DECR year END IF FUNCTION = (((year \ 100) * %DAYS_PER_400_YEARS) \ 4 _ + ((year MOD 100) * %DAYS_PER_4_YEARS) \ 4 _ + (month * %DAYS_PER_5_MONTHS + 2) \ 5 _ + iDay _ - %JDN_OFFSET) END FUNCTION '============================================================================= ' FUNCTION DayOfWeek(BYVAL jdn AS LONG) AS LONG ' ' Convert a JDN to a day-of-week number (0 to 6). Where 0 stands for ' Sunday, 1 for Monday, etc. and 6 stands for Saturday. '============================================================================= FUNCTION DayOfWeek(BYVAL Jdn AS LONG) AS LONG ' correction pointed out by Don Schullian ' the following seems to work for all dates that I have tested. FUNCTION = (JDN + 1) MOD 7 ' no need to test for negative numbers here, the only way to get ' negative numbers would be if JDN were passed in < 0 which would ' be an error. ' original "C" to PB translation 'DIM dow AS LOCAL LONG ' 'dow = (Jdn + 1) MOD 7 'IF (dow >= 0) THEN ' FUNCTION = dow 'ELSE ' FUNCTION = dow + 7 'END IF END FUNCTION

[This message has been edited by Errol Cheverie (edited November 25, 2002).]

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