pw-schedule: Use locale-independent date format for days

Having en_GB.UTF-8 as the default locale on my system, pw-schedule fails
to run with the following error:

    $ ./pw-schedule
    date: invalid date ‘Mon 18 Jul 10:11:30 BST 2022’

Apparently, "date" is not able to parse the date it generated in that
locale.

To fix this, use the same, locale-independent format for days
everywhere: +%m/%d/%Y.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>