[lowercaseman]

Found this: https://github.com/jbenet/nanotime. No idea if related in any way or used for APFS but it was interesting enough for a quick read. This one is MIT licensed. A little more searching I found a variant but it's done with big-endian so it's much different.

[achivetta]

64-bit nanosecond values are actually pretty common on Darwin already. See e.g. clock_gettime_nsec_np(3).