shellDatediff

Calculate time differences with shell builtins.

Shell utility for calculating time intervals between dates. Works with Ksh, Bash and Zsh.

The project contains a small shell library to calculate elapsed time between dates as compound time ranges and as single-unit fractions - all while handling timezone offsets.

The script works with ISO-8601 and UNIX timestamps directly. It can optionally leverage C-code date to process diverse date formats as input.

Beyond time intervals, it offers a few helpful calendar functions for day-to-day use.

✨ Features

• Date input as ISO-8601, or UNIX time
• Optionally warps C-code date to parse various date formats
• Timezone offset aware, heeds environment $TZ
• Check whether year is leap
• Calculate moon / lunar phases
• Calculate Easter, Carnaval, and Corpus Christi dates
• Check for next Friday the 13th (or any day-of-week/month combination)
• Stdin input (pipe) support

Usage Examples

Time elapsed between two dates

% datediff.sh -u  2008-01-15  2024-09-11

DATES
2008-01-15T00:00:00+00:00       1200355200
2024-09-11T00:00:00+00:00       1726012800
RANGES
16Y 07M 03W 06D  00h 00m 00s
16.7 years | 199.9 months | 869.1 weeks | 6084.0 days | 146016.0 hours | 8760960.0 mins | 525657600 secs

When only one date is specified, the first date is assumed to be now.

Setting option -u performs all date calculations in UTC. It also influences how the underlying C-code date programme processes dates.

When the last argument of the command line is exactly y, mo, w, d, m, or s, only the specified time frame result unit is calculated.

Alternatively, set options -vvv to filter the main output layout for specific fields.

For example, calculate the compound time range only:

% datediff.sh -vv  2025-03-30T12:33:58  2031-04-17T04:34:10

6Y 00M 02W 03D  16h 00m 12s

Mind that input dates must be ISO-8601 or UNIX time. When available, C-code date is leveraged to parse user input in various date formats.

Result layout

The main function is very verbose by default and prints two sections with processed dates (DATES) and time range results (RANGES).

The user can filter out which fields are going to be calculated and printed.

Set the verbose option -v up to three times to select different layouts in the main function. Setting -v in other functions decreases verbose.

Set option -v once to print all single unit results only:

% datediff.sh -v  2008-01-15

17.4 years | 209.3 months | 910.3 weeks | 6371.8 days | 152923.3 hours | 9175400.5 mins | 550524032 secs

Note: if only one date is specified, the first date is assumed to be now. Examples in this group run on 2025-06-25.

Compound time range (AST date style):

% datediff.sh -vvv  2008-01-15

17Y05M01W03D01h00m00s

Single time unit result

The user can optionally set the last positional parameter as exactly y, mo, w, d, m or s to print only the specific single-unit result:

% datediff.sh  2008-01-15  2025-06-25  mo

209.3 months

Decimal plates

The number of decimal plates shown in float results can be set with option -[num], where num is an integer. For three decimal plates, the incantation should start as datediff.sh -3.

Results are subject to rounding for improved precision!

Table view

There is also a table layout with single-unit results. This is activated with option -t.

To print results in the table layout, set options -tt at the command line incantation:

% datediff.sh -3 -t -u  2008-01-15  2025-06-25

Years          17.440
Months        209.323
Weeks         910.143
Days         6371.000
Hours      152904.000
Mins      9174240.000
Secs    550454400

Check when next Friday the 13th is:

Using the current date by default (run on 2025-06-25):

% datediff.sh -F  Fri 13

Fri, 13 Feb 2026 is  233 days away

Check any combination of day-in-week and day-in-month:

Optionally specify a start date for the search.

% datediff.sh -F  Mon 1  2030-04-10


Mon, 01 Jul 2030 is   82 days away

Set options -FF to print the following 10 date matches as a list!

Check whether a year is leap

% datediff.sh -l  2032

leap year -- 2032

The exit code is 1 if a year is not leap.

Set option -v to decrease verbose.

Generate lunar phase calendar

% datediff.sh -m  2030-01

2030-01-01  Waning Crescent
2030-01-03  New Moon
2030-01-07  Waxing Crescent
2030-01-10  First Quarter
2030-01-14  Waxing Gibbous
2030-01-18  Full Moon
2030-01-21  Waning Gibbous
2030-01-25  Last Quarter
2030-01-29  Waning Crescent

For multiple-month calendar:

% datediff.sh -m  2030-{01..12}

#OR

% datediff.sh -m  2030

Setting option -m without an argument shows the moon phase for current date.

Compute dates of Carnaval, Easter and Corpus Christi

% datediff.sh -ee  2030

  Carnaval          Easter      CorpusChristi
2030-03-05      2030-04-21      2030-06-20

Set multiple years to calculate a table of dates:

% datediff.sh -ee  20{23..30}
  Carnaval          Easter      CorpusChristi
2023-02-21      2023-04-09      2023-06-08
2024-02-13      2024-03-31      2024-05-30
2025-03-04      2025-04-20      2025-06-19
2026-02-17      2026-04-05      2026-06-04
2027-02-09      2027-03-28      2027-05-27
2028-02-29      2028-04-16      2028-06-15
2029-02-13      2029-04-01      2029-05-31
2030-03-05      2030-04-21      2030-06-20

The dates are for the Western Church.

More Examples

Check further examples at the man page.

Dependencies

• Ksh93, Bash, or Zsh
• GNU/BSD/AST/Busybox date (optional)
• Basic Calculator bc and Desk Calculator dc (optional)

Debugging dependencies

• datedff.debug.sh script
• Hroptatyr's C-code datediff

Help

Please, check script help page with datediff.sh -h or the online man page.

Project Source

• GitLab https://gitlab.com/fenixdragao/shelldatediff
• GitHub https://github.com/mountaineerbr/shellDatediff

Extensively tested, see testing scripts, notes, and man page.

See Also

• C-code dateutils/datediff, Hroptatyr, http://www.fresse.org/dateutils/.
• Python PDD, Jarun, https://github.com/jarun/pdd.
• AST date, see elapsed time option -E, https://github.com/att/ast.
• GNU units, https://www.gnu.org/software/units/.
• ``Calendrical calculation'', Dershowitz and Reingold, 1990, http://www.cs.tau.ac.il/~nachum/papers/cc-paper.pdf.