We had a spark user ( Spark 2.5.4, Openfire 3.3.1 ) that had a time problem. The times on his old and new IM Message times were 7 hours off. His time zone was set to (GMT-8:00) Tijuana, Baja California.
Changing the time zone to (GMT-08:00) Pacific Time (US & Canada), and then restarting Spark corrected all the IM Message times.
I reproduced this on my 2.5.4 client and get the same behavior.
Sounds like Spark does not like Mexico