That is using the ternary operator to set the amount of time we want to subtract when creating the datestamp. You can read up on the ternary operator in 'perldoc perlop'.
36000 is the number of seconds in 10 hours. Another monk used 60*60*24 which == 86400 and is the number of seconds in 24 hours.
I was going to use the 86400 value, but in this case 36000 should be sufficient to roll the datestamp back to the previous day. If the hour is grater than 10, then there's no need to adjust the datestamp, so $minus is set to 0.