julian-day-converter

Shell scripts to convert a date in ISO format to a Julian Day Number and back again

Introduction

A Julian Day Number is similar to a Unix timestamp, counting days instead of seconds, and using January 1st of the year 4713 BC as starting point (or "epoch").

This is a useful format for computations on dates:

• adding a number of days to a date to get a new date
• subtracting dates to get the interval in days

Algorithms

The conversions use the algorithms described on Wikipedia:

1. [Converting Julian or Gregorian calendar date to Julian Day Number] (https://en.wikipedia.org/wiki/Julian\_day#Converting\_Julian\_or\_Gregorian\_calendar\_date\_to\_Julian\_Day\_Number)
2. [Gregorian calendar from Julian day number] (https://en.wikipedia.org/wiki/Julian\_day#Gregorian\_calendar\_from\_Julian\_day\_number)

Usage

The script iso2julian.sh converts an ISO date in format yyyy-mm-dd to a Julian Day Number:

$ ./iso2julian.sh 2015-03-18
2457100

The script julian2iso.sh converts a Julian Day Number back to an ISO date in format yyyy-mm-dd:

$ ./julian2iso.sh 2457100
2015-03-18

The date to convert can be provided as parameter; otherwise, it will be read from standard input:

$ echo '2015-03-18' | ./iso2julian.sh
2457100

$ echo 2457100 | ./julian2iso.sh
2015-03-18

In both cases, the converted date is written to standard output.

License

CC0