datemath - manipulate dates mathematically

Too many times we have needed to do a bit of math on a date -- say, "today + 7 days" -- but in traditional MM/DD/YYYY format this is really tricky (especially in a shell script). A result of this need we built the datemath tool which can perform these functions from the command line or from a shell script. Examples:

$ datemath today + 5
06/23/2003

$ datemath '12/25/2003 - today'
188

$ datemath today + 5 weeks
07/25/2003

when will my machine be up for one year?
$ uptime
 11:09am  up 317 days, 15:38,  7 users,  load average: 0.16, 0.04, 0.01
$ datemath today + 365 - 317
10/24/2003

We've been using this for 15 years, and recently got around to porting it and updating for year-2000 compatibility (oops). The port to Bison/flex from yacc/lex was a bit of doing as well.

Date expressions accepted

The expressions accepted by datemath are straightforward and moderately flexible. The grammer is defined to reject most things that don't make any sense (such as adding two dates together), while allowing most things that do.

Dates can be entered in one of three formats:

mm/dd/yy - in two-digit format, the full year is guessed based on the cutoff defined with -9 parameter. The default cutoff is "40", so 1/1/40 is in 1900, but 1/1/39 is 2039.
mm/dd/yyyy - the four-digit year is accepted without ambiguity
yyyy/mm/dd - This is the "collated date" format that lends itself to sorting, and it only works with a four-digit year.

Keywords are shown here in upper case to make them stand out, but the program doesn't care at all about case, and mixed upper/lower work fine. Remember that parentheses are special to the shell, so they must be quoted.

TODAY a synonym for today's date

YESTERDAY a synonym for TODAY-1

TOMORROW a synonym for TODAY+1

FDAY(date) Returns the date of the first day of the month 1..31

LDAY(date) Returns the date of the last day of the month 1..31

DAY(date) Return the day of the month 1..31

MONTH(date) Returns the number of month 1..12

YEAR(date) Returns the year in four-digit form

NDAYS(date) Returns the number of days in the given month 1..31

n WEEKS Returns the number of days in n weeks

date + ndays Return the date ndays into the future

date - ndays Return the date ndays into the past

date - date Return the number of days between the two dates

These can be combined in most combinations that make sense, though we've never gotten too elaborate about making sure that it's completely orthogonal.

Command Line Parameters

In addition to the date expression above (required),

-V This displays the program's version information, and exits.

-4 Display dates with four-digit years (the default)

-2 Display dates with two-digit years

-C (Collation mode) Display dates in year/month/day format that is useful for sorting.

-9 ## When interpreting a date with a two-digit year, years from ##..99 are 1900, otherwise 2000. Default value = 40.

Bugs

This progarm was written in about 1987 when the year 2000 was far in the distance: it's likely that it's not been completely retrofitted in every case.
Not all things that makes sense are implemented. For instance, the notation
```
1/31/2003 + 3 MONTHS
```
doesn't work because it's not quite clear what it means: 4/31/2003 is not a valid date, so we couldn't find any obvious direction to adjust the date to conform.
The port from lex/yacc to flex/bison was probably done poorly, and it's likely that it won't "back port" to the older tools easily.

Download

The source is delivered as a gzip'd GNU tar archive (do a "save as"):

datemath.tar.gz - compressed tarball of source

The files inside are:

$ gtar -tzf datemath.tar.gz
datemath.c
gram.y
lex.l
util.c
jdate.c
defs.h
Makefile

Note that this package requires GNU flex and bison: those needing to use the older tools may have some troubles. Reports of portability issues are gratefully accepted.

Version History

Fri Jun 20 23:01:46 PDT 2003 -- version 2.0.10
-- initial release to website

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TODAY	a synonym for today's date
YESTERDAY	a synonym for `TODAY-1`
TOMORROW	a synonym for `TODAY+1`
FDAY(date)	Returns the date of the first day of the month 1..31
LDAY(date)	Returns the date of the last day of the month 1..31
DAY(date)	Return the day of the month 1..31
MONTH(date)	Returns the number of month 1..12
YEAR(date)	Returns the year in four-digit form
NDAYS(date)	Returns the number of days in the given month 1..31
n WEEKS	Returns the number of days in n weeks
date + ndays	Return the date `ndays` into the future
date - ndays	Return the date `ndays` into the past
date - date	Return the number of days between the two dates

-V	This displays the program's version information, and exits.
-4	Display dates with four-digit years (the default)
-2	Display dates with two-digit years
-C	(Collation mode) Display dates in year/month/day format that is useful for sorting.
-9 ##	When interpreting a date with a two-digit year, years from ##..99 are 1900, otherwise 2000. Default value = 40.