I have a date file which contains logon and logoff times. My task is to work out the time online. To do this have decided to use the date program with the +%s option to give the time since epoch. Date needs the input date to be in the form 2003-03-04 13:45:00, however my data is in the form 2003:03:04:13:45:00. How can I change the format of the live into something which i can pipe into date? To make things more complicated I cannot use awk, grep of sed.