Greetings Paul,
'show interfaces link' will show you the total time that each interface spent in a down state along with the timestamp when it last changed state to down. Currently up interfaces won't show anything there however.
You could also check the ifLastChange SNMP object which records the sysUpTime value when each interface entered it's current operational state, so that'd give you when a currently up interface last came up.
To get the complete history, the best option is enabling 'logging event link-status' per-interface or 'logging event link-status global' in global for all interfaces and inspecting the system log for LINK-3-UPDOWN messages, either on the device or by exporting it to an external syslog server. In that case, consider increasing the size of the logging buffer and adjusting the timestamp format to make the logs more human friendly, for example 'service timestamps log datetime localtime msec'.
Hope this helps!
/Phil
