Why?
We are a global forum.
Time stamping based on UTC (Universal Time, Coordinated) in the PM's obviates the need to know the location of the sender or recipient to determine when a message or response was sent or responded.
For example, if I send a message from CO at 0630 MDT (UTC 1230H) to a recipient in London who responds at local time of 1330H (UTC 1330H), the UTC time stamps will reflect that the recipient replied in 1 hour, not 7 hours. I do not need to know, or may not know, that the response was from London.