To answer one of your questions, the time frame covered in the "show interface" command is the cumulative time since some someone manually cleared the counters with the "clear counters serial0/0/0"(replace serial0/0/0 with the interface you are interested in) command or the router was reloaded. To calculate the number of errors per time period, use that command to clear the counters and note the current time. Then after a suitable period, run the "show interface" command. Lets say you waited 5 minutes after clearing the counters before you run "show interface" then the data shown is for the last 5 minutes.

Also, see this article and pay particular attention to the sections on clocking:
http://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1915.html