# perl -e 'print time' will return the current time in Unix epoch format

# So if you substract 43200, that should give you the current time minus 30 minutes in Unix epoch format:

time=$(perl -e 'print time-43200')

##</code><code>##

c:\@Work\Perl\monks>perl -wMstrict -le
"my $t = time;
 print $t;
 print scalar localtime $t;
 print scalar localtime $t - 43200;
 print scalar localtime $t -  1800;
"
1597857155
Wed Aug 19 13:12:35 2020
Wed Aug 19 01:12:35 2020
Wed Aug 19 12:42:35 2020