UTCTime (ZigBee)

[1] DataTypes (ZigBee 2.0 API) (2014-01-09 22:06:38 +09:00) <http://dz.prosyst.com/pdoc/mBS_SH_SDK_7.4.1/modules/zigbee/api/com/prosyst/mbs/services/zigbee/datatype/DataTypes.html>

ZigBee UTC time - passed seconds since 1st Jan 2000.

[2] python - Conversion from UNIX time to timestamp starting in January 1, 2000 - Stack Overflow (2017-05-11 02:07:46 +09:00) <http://stackoverflow.com/questions/35763357/conversion-from-unix-time-to-timestamp-starting-in-january-1-2000>

I found the following datatype entry in their documentation:
1.3.2.7 Absolute time
This is an unsigned 32-bit integer representation for absolute time. Absolute time is measured in seconds from midnight, 1st January 2000.