The date command on Linux can be used to see the current date and time, but we can also use addition and subtraction arithmetic with the command to extend its functionality. For example, instead of seeing the current date, we can see the date and time from five days ago, five years in the future, etc. The possibilities here are endless.
This is a companion discussion topic for the original entry at https://linuxconfig.org/addition-and-subtraction-arithmetics-with-linux-date-command