A Timestamp represents a point in time independent of any time zone or calendar,
represented as seconds and fractions of seconds at nanosecond resolution in UTC
Epoch time. It is encoded using the Proleptic Gregorian Calendar which extends
the Gregorian calendar backwards to year one. It is encoded assuming all minutes
are 60 seconds long, i.e. leap seconds are "smeared" so that no leap second table
is needed for interpretation. Range is from 0001-01-01T00:00:00Z to
9999-12-31T23:59:59.999999999Z. By restricting to that range, we ensure that we
can convert to and from RFC 3339 date strings.