Change timefile calculations to respect slack

Change timefile calculations to respect slack

% touch -t $(date -d '7 days ago' +%Y%m%d0000) somefile
% ls -l somefile
-rw-r--r-- 1 henk henk 0 Dez 16 00:00 somefile

Before this change:
% /usr/bin/snooze -nv -s 30d -H 5 -t somefile -T 30m
2020-12-23T05:00:00+0100 Wed  0d  4h 32m 24s
2020-12-24T05:00:00+0100 Thu  1d  4h 32m 24s
2020-12-25T05:00:00+0100 Fri  2d  4h 32m 24s
2020-12-26T05:00:00+0100 Sat  3d  4h 32m 24s
2020-12-27T05:00:00+0100 Sun  4d  4h 32m 24s

After this change:
% ./snooze -nv -s 30d -H 5 -t somefile -T 30m
2020-12-16T05:00:00+0100 Wed -6d-19h-27m-43s
2020-12-17T05:00:00+0100 Thu -5d-19h-27m-43s
2020-12-18T05:00:00+0100 Fri -4d-19h-27m-43s
2020-12-19T05:00:00+0100 Sat -3d-19h-27m-43s
2020-12-20T05:00:00+0100 Sun -2d-19h-27m-43s