Ygtjki

I understand the command date -d 'last-monday - 14 days' +%Y%m%d will print the Monday two weeks ago from "Today's date". I need a way to test this for different dates and see the result. Almost like I need to mention the date command to do calculations off a relative date.

I need something to test with different dateslike :

date -d 'last-monday - 14 days' %Y%m%d from 20190315

date -d 'last-monday - 14 days' %Y%m%d from 20180217

date -d 'last-monday - 14 days' %Y%m%d from 201700914

and see the respective outputs.

#!/bin/bash

# Our given dates
dates=(
 20190315
 20180217
 20170914
)

# Loop over the given dates
for thedate in "$dates[@]"; do
 # Get day of the week as digit (1 is Monday, 7 is Sunday)
 day=$( date -d "$thedate" +%u )
 # The Monday the same week is $(( day - 1 )) days earlier than the given date.
 # The Monday two weeks earlier is 14 days earlier still.

 date -d "$thedate -$(( day - 1 + 14 )) days" +"$thedate --> %Y%m%d"
done

Output:

20190315 --> 20190225
20180217 --> 20180129
20170914 --> 20170828

The difficult bit about this is to figure out how to construct the correct --date or -d string for GNU date to compute the final date. I opted for computing the day of week of the given date, and then using that to compute a date string that offsets the given date by a number of days so that the resulting date is the Monday two weeks earlier.

The actual strings that ends up being used for the option argument to -d in the above script, using the dates given in the script, are

20190315 -18 days
20180217 -19 days
20170914 -17 days

Condensing the script into a single command that does the computation for a single date in $thedate:

date -d "$thedate -$(date -d "$thedate" +%u) days -13 days" +%Y%m%d

or

date -d "$thedate -$(date -d "$thedate" +"-%u days -13 days")" +%Y%m%d

On at least Debian there is a faketime package

faketime '2019-03-15' date
Fri 15 Mar 00:00:00 GMT 2019

faketime '2019-03-15' date --date 'last monday - 14 days'
Mon 25 Feb 00:00:00 GMT 2019

$ date
Wed Mar 20 15:02:23 MST 2019
$ date -d "last-monday"
Mon Mar 18 00:00:00 MST 2019
$ date -d "last-monday - 1 week"
Mon Mar 11 00:00:00 MST 2019

The overkill solution:

Create a bare-bones virtual machine. 
Run it in single user mode,
so no background services are running. 
Just to be sure, double-check that NTP is not running.

Then set the date to whatever you want to set it to,
and do your tests.

Use the date command's -s/--set option:

# date
Wed Mar 20 23:02:18 CET 2019
# date -s '20190315' > /dev/null; date -d 'last-monday - 14 days'
Mon Feb 25 00:00:00 CET 2019

Depending on your system, you might have to prefix with sudo or reset afterwards via ntpdate