CCNx Timestamps are expressed in units of 2-12 seconds since the start of Unix time, using the minimum number of bytes. The time portion will thus take 48 bits until quite a few centuries from now (Sun, 20 Aug 4147 07:32:16 GMT, if you must know), at which point an additional byte will be required. With 12 bits of fraction, this format allows for sub-millisecond resolution.
Note that this format does not exactly represent even numbers of milliseconds. Some rounding is required for conversions from/to common time representations.
The CCNx Timestamp format is used for: