Thinking 8 bytes and ticks you might want to try the following:

First four bytes (0-3)    |  Last four bytes (4-7)
---------------------------------------------------
seconds since epoch,      |  additional micro-
e.g. 00:00 1st Jan 1970   |  seconds

Both four byte values are 32 bit integers.
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However, my first calculations give timestamps nowhere near the ones you provide. Maybe some CCSDS time format was used?