Mark REV values as UTC (see #41)

[rfc2822]

No description provided.

[skjnldsv]

We need to be sure we're following rfc 3339

Z           A suffix which, when applied to a time, denotes a UTC
            offset of 00:00; often spoken "Zulu" from the ICAO
            phonetic alphabet representation of the letter "Z".

https://www.ietf.org/rfc/rfc3339.txt

[zeehio]

I am all for the "Z".

Dropping the milliseconds does not go against the rfc3339. In fact:

5.3. Rarely Used Options

   A format which includes rarely used options is likely to cause
   interoperability problems.  This is because rarely used options are
   less likely to be used in alpha or beta testing, so bugs in parsing
   are less likely to be discovered.  Rarely used options should be made
   mandatory or omitted for the sake of interoperability whenever
   possible.

   The format defined below includes only one rarely used option:
   fractions of a second.  It is expected that this will be used only by
   applications which require strict ordering of date/time stamps or
   which have an unusual precision requirement.

So either there is a rationale for keeping milliseconds or we avoid them.