civil_date.nx

civil_date.nx -- Gregorian civil date from unix seconds. Pairs with ntp.nx (returns unix seconds) to convert seconds into (year, month, day, hour, minute, second, weekday) without depending on glibc strftime or the OS tzdata. UTC only; timezone handling is a separate concern. Uses Howard Hinnant's "civil_from_days" algorithm (the <chrono> library implementation used by libc++). Correct for all years in i64 range (billions of years past and future). Algorithm: z = days since 1970-01-01 Shift epoch to 0000-03-01 so Feb is last: z += 719468 era = z / 146097 (146097 days = 400 years) doe = z % 146097 yoe = (doe - doe/1460 + doe/36524 - doe/146096) / 365 y = yoe + era*400 doy = doe - (365*yoe + yoe/4 - yoe/100) mp = (5*doy + 2) / 153 d = doy - (153*mp + 2)/5 + 1 m = mp < 10 ? mp + 3 : mp - 9 y += m <= 2 ? 1 : 0 Invariants: CD1 1970-01-01 00:00:00 UTC <-> unix_sec = 0, all fields returned accordingly (year=1970, month=1, day=1, ...). CD2 Negative unix_sec works (dates before 1970). CD3 Weekday: 0 = Sunday, 1 = Monday, ..., 6 = Saturday (Unix convention; struct tm compatible).