DateSum v1.00 build 2010280 - Simple day conversion calculator for dates
copyright (c) bytepointer.com 2020

syntax:
  datesum [options] <start_date> [end_date]

[options]
  -a <days>         action add: add days to start_date
  -s <days>         action sub: subtract days from start_date
  -j                output julian-day result
  -u                output gregorian date result in US format: MM-DD-YYYY
  -q                quiet; output concise result

usage:
  Calculate days between <start_date> and [end_date], where dates may appear
  in any order. If [end_date] is omitted, perform an arithmetic action on
  <start_date>. When no action is specified, the default action is to convert
  between julian-day integers and human-readable gregorian-calendar dates.

  Dates are specified in Y-M-D format or as positive julian-day integers+
  (without hyphens). Special date keywords are additionally supported:
  "today" and/or "yesterday" to reference current system time values.

  Results are output in YYYY-MM-DD format by default, but this can be modified
  with the -u and -j options below.

+ this tool uses an astronomical julian-day algorithm representing days since
  the base date of Nov 24, 4713 (BC) thru full years up to 1,465,001 (AD).
  This allows julian-days ranging from 0 to 536802342. Only whole days are
  supported, thus there is no concept of time-of-day or fractional julian-
  day values as found with similar algorithms.

examples:
  -display the number of days since the 2016 Winter Solstice:
    > datesum 2016-12-21 today

  -display the julian-day for Halloween of 2020:
    > datesum -j 2020-10-31

  -what is 90 days from yesterday:
    > datesum yesterday -a 90

  -what is the date 3 weeks before Christmas of 2020:
    > datesum 2020-12-25 -s 21