​
Summary

  1. /timediff (+ reply) - time diff between now and replied message
  2. /timediff [message_id] - time diff between now and message ID
  3. /timediff [message_id] (+ reply) - time diff between message ID and replied message
  4. /timediff [message_id] [message_id] - time diff between 2 message IDs

​
Usage 1

No arguments provided, but you need to reply to the message. It will tell you the difference between now and the timestamp of the replied message 
/timediff (+ reply)

​
Usage 2

Only 1 argument provided, as the message ID. It will tell you the difference between now and timestamp of the message. 
/timediff <message_id>

​
Usage 3

Only 1 argument provided, as the message ID. You need to reply to a message, and it will give you the time difference between the message ID and the timestamp of the replied message. 
/timediff <message_id> (+ reply)

​
Usage 4

2 arguments provided. It will tell you the difference between the two messages. 
/timediff <message_id> <message_id>