2024-12-24
Thoughts on practical date/time formats
solyga@gmx.de

requirements
1 resolution at least 1ms
2 date representation around 'now' shall be exact


(well) known formats
a seconds since 1970-01-01
  -> breaks req 1
b date = days since minYear-01-01, time = milliseconds since midnight
  1 maxTime (+1) = 24 * 3600 * 1000 = 86400000 = 5265C00H
    -> CARD32 for req 1
  2 CARD16 allows for 10000H / 365.2425 = 179 years
    -> too few for some general format
    -> CARD32 needed
  -> 8 bytes for date/time required
c days since chosen reference day as LONGREAL, fraction represents time
  -> resolution at (start of) reference day = 2^(-52) d = 2.22E-16 * 86400 s =
    = 19.185E-12 s = 19.185 ps
  -> resolution 2000 years (= 730485 days) later: 730485 * 2^(-52) d =
    = 1.4014E-5 s = 14.014 µs
  1 Pascal epoche: refDay = 1899-12-30 (two days off, thx to Lotus et al)
    resolution on 2024-12-24 (124 y = 45290 d): 45290 * 19.185E-12 s =
    = 8.6887E-7 s = 0.86887 µs
  2 Christ epoche: refDay = 0000-01-01 (many days off - in nearly one million days)
    resolution on 2024-12-24 (2024 y = 739616 d): 739616 * 19.185E-12 s =
    = 1.4189E-5 s = 14.189 µs



calendarian rules
A 1d = 24h, 1h = (60min, 1min = 60s) 3600s (1d = 86400s, no leap seconds)
B IsLeapYear( y ) :=
    ( y MOD 4 = 0 ) & ( y MOD 100 # 0 ) OR ( y MOD 400 = 0 )
C DaysInYear( y ) :=
    365 + ORD( IsLeapYear( y ) )
D DaysInMonth( m, y ) :=
    IF m = 2 THEN 28 + ORD( IsLeapYear( y ) ) ELSE 31 - ( ( m - 1 ) MOD 7 ) MOD 2 END
E 1990-01-01 was a Monday
F weekday rule...