My earlier post, Fun with Date Arithmetic shows how to use the date command to compute a future or past date that is a certain number of days ahead or in the past. This post expands on how you can manipulate dates on the Linux command-line interface using the cal and dateutils programs. These commands can give you results that GUI calendars can't.
cal
While the date command works with the basic unit of days, say display the day that is 3 days from today, the cal command manipulates months, say display the month that is 2 months away.
Running cal without any argument displays the calendar for the current month.
$ cal
July 2020
Su Mo Tu We Th Fr Sa
1 2 3 4
5 6 7 8 9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29 30 31
While GUI calendar programs are well capable of displaying single month calendars, they are no match to the cal command for simultaneously displaying multiple months.
To display the current and the following say 2 months, use the -A2 argument:
$ cal -A2
2020
July August September
Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa
1 2 3 4 1 1 2 3 4 5
5 6 7 8 9 10 11 2 3 4 5 6 7 8 6 7 8 9 10 11 12
12 13 14 15 16 17 18 9 10 11 12 13 14 15 13 14 15 16 17 18 19
19 20 21 22 23 24 25 16 17 18 19 20 21 22 20 21 22 23 24 25 26
26 27 28 29 30 31 23 24 25 26 27 28 29 27 28 29 30
30 31
To display past months is equally easy with the -B argument. In fact, you can combine -B and -A to, for example, display the previous and the next month as follows:
$ cal -A1 -B1
June 2020 July 2020 August 2020
Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa
1 2 3 4 5 6 1 2 3 4 1
7 8 9 10 11 12 13 5 6 7 8 9 10 11 2 3 4 5 6 7 8
14 15 16 17 18 19 20 12 13 14 15 16 17 18 9 10 11 12 13 14 15
21 22 23 24 25 26 27 19 20 21 22 23 24 25 16 17 18 19 20 21 22
28 29 30 26 27 28 29 30 31 23 24 25 26 27 28 29
30 31
cal has a special shortcut for displaying the above combination, namely, the current month with the immediate before and after month:
$ cal -3
dateutils
While date is a nifty program to do date arithemetic, dateutils is a more versatile collection of tools for date manipulation.
To install the dateutils program on Debian,
$ sudo apt install dateutils
dateutils.dadd
Use the dateutils.dadd sub-command to do date arithmetic. The program takes 2 input: a date and a duration either before or after the said date, specified in years, months, weeks, or days. For instance, to output the date that is 1 week and 2 days in the future from today, say 2020-07-10:
$ dateutils.dadd today +1w2d
2020-07-19
You can replace the special keyword today with a specific date. For instance, to compute the date that is 1 year, 2 months and 3 days in the past from 2020-07-10:
$ dateutils.dadd 2020-07-10 -1y2m3d
2019-05-07
dateutils.ddiff
Given 2 dates, dateutils.ddiff computes the duration between them. The default output unit is the number of days. You can customize the output units by specifying a format using the -f argument.
$ dateutils.ddiff 2020-07-02 2020-08-21 -f "%y years %m months %w weeks %d days"
0 years 1 months 2 weeks 5 days
