A UTC timestamp for use in Holochain’s headers. It is assumed to be untrustworthy: it may
contain times offset from the UNIX epoch with the full +/- i64 range. Most of these times are
not representable by a chrono::DateTime (which limits itself to a +/- i32 offset in days
from Jan 1, 0AD and from 1970AD). Also, most differences between two Timestamps are not
representable by either a chrono::Duration (which limits itself to +/- i64 milliseconds), nor
by core::time::Duration (which limits itself to +’ve u64 seconds). Many constructions of these
chrono and core::time types will panic!, so painful measures must be taken to avoid this outcome
– it is not acceptable for our core Holochain algorithms to panic when accessing DHT Header
information committed by other random Holochain nodes!