From what I can tell, these are unix timestamps in milliseconds. sent-ts is equivalent to Sat, 18 Jul 2015 20:08:40 GMT and tmi-sent-ts is basically the same thing but off by a few milliseconds. I suppose this can be useful for checking ping but I’ve no clue why TMI would be sending these.
Edit: Also, this is what I used to convert them. Neat thing about this site is that it will tell you if it’s a timestamp in milliseconds.