handle negative timestamps on Windows

[jsta]

See https://issuetracker.google.com/issues/191997926#comment4

[jsta]

The current and proposed code give identical outputs on Linux but only the proposed code works on Windows:

import datetime

timestamp_ms = -50000

# current 
datetime.datetime.utcfromtimestamp(timestamp_ms / 1000.0).isoformat() + "Z"

# proposed
(
    datetime.datetime.utcfromtimestamp(0)
    + datetime.timedelta(seconds=timestamp_ms / 1000)
).isoformat() + "Z"

[jsta]

This allows for handling dates pre-1970