Apologies for late reply.
You can send the current time along with the message so that when you receive the message at the other end, you can calculate the time elapsed in between. UTC time stamp means the time as per the UTC time zone. You should send current time in UTC format because your player may be from different time zones so comparing the from two different time zones will be difficult. But UTC time will be same for both the players so you can easily get the correct time difference.
Please let me know if you have any further queries.