Support for Data Time Stamps

The timestamps (type: timeticks) provided by SNMP are based on sysUpTime (see MIB-II), which is relative to the device restart rather than any useful epoch. Further, sysUpTime will roll over eventually.

To provide for tag values, and other useful timestamps, the SNMP Agent encodes timestamps in type Unsigned32 as the number of seconds since midnight of January 1, 1970 (where "midnight" is 00:00). By convention all timestamps will be in UTC.