How do I do date math in a bash script on OS X Leopard?

Question

I realize I could whip up a little C or Ruby program to do this, but I want my script to have as few dependencies as possible.

Given that caveat, how does one d

Answer 1

I used the following on, macOS Mojave 10.14.5, to find the DHCP renewal time open

date -v +`ipconfig getoption en0 renewal_t1_time_value`S