Home » Quickly calculate date differences

# Quickly calculate date differences

## Solutons:

The “n weeks after a date” is easy with GNU date(1):

``````\$ date -d 'now + 3 weeks'
Tue Dec  6 23:58:04 EST 2011
\$ date -d 'Aug 4 + 3 weeks'
Thu Aug 25 00:00:00 EST 2011
\$ date -d 'Jan 1 1982 + 11 weeks'
Fri Mar 19 00:00:00 EST 1982
``````

I don’t know of a simple way to calculate the difference between two dates, but you can wrap a little logic around date(1) with a shell function.

``````datediff() {
d1=\$(date -d "\$1" +%s)
d2=\$(date -d "\$2" +%s)
echo \$(( (d1 - d2) / 86400 )) days
}
\$ datediff '1 Nov' '1 Aug'  # Note: answer should be 92 days but in my timezone, DST starts between the dates.
91 days
``````

Swap `d1` and `d2` if you want the date calculation the other way, or get a bit fancier to make it not matter. Furthermore, in case there is a non-DST to DST transition in the interval, one of the days will be only 23 hours long; you can compensate by adding ½ day to the sum.

``````echo \$(( (((d1-d2) > 0 ? (d1-d2) : (d2-d1)) + 43200) / 86400 )) days
``````

For a set of portable tools try my very own dateutils. Your two examples would boil down to one-liners:

``````ddiff 2011-11-15 2012-04-11
=>
148
``````

or in weeks and days:

``````ddiff 2011-11-15 2012-04-11 -f '%w %d'
=>
21 1
``````

and

``````dadd 2011-11-15 21w
=>
2012-04-10
``````

Important note regarding Ubuntu installation:

These very same dateutils are available as a Ubuntu package, and hence installable through `sudo apt install dateutils`.

However, commands need to be preceded by a `dateutils.` prefix, as in `dateutils.ddiff 2019-03-28 2019-05-16`

They are also available in Fedora and in Fedora EPEL for RHEL or CentOS with `sudo dnf install dateutils`. In Fedora, these packages do not require a prefix but use the long names — e.g, `datediff` and `dateadd` instead of `ddiff` and `dadd`.

For Arch Linux, install the package with `sudo pacman -Sy dateutils`. The binaries are called `datediff` and `dateadd`, just like in Fedora.

A python example for calculating the number of days I’ve walked the planet:

``````\$ python
>>> from datetime import date as D
>>> print (D.today() - D(1980, 6, 14)).days
11476
``````

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