man 5 shadow gives the following description of the third field in each line:
The date of the last password change, expressed as the number of days since Jan 1, 1970.
The value 0 has a special meaning, which is that the user should change her pasword the next time she will log in the system.
An empty field means that password aging features are disabled.
Does this refer to times in UTC or the local timezone? In particular, if I want to calculate comparable numbers using something like
$(( $(date +%s) / 86400 )), do I need to pass