>> The timestamp is the number of seconds since 1st January 1970. If we convert 1563188141 to more human date we'll get "2019-07-15 10:55:41" — that is the time in UTC timezone. Then we add "03" hours and "00" minutes to that time and get "2019-07-15 13:55:41" — that is the time the commit author can see on his wall clock when he did commit.
Usually the +0300 indicates that the times tamp is at +3 hours from UTC. Eg. the timestamp is already in local wall time. The offset can be used to convert it to UTC.
So if the author did what they wrote. Then I reckon all the data is actually in UTC and not local wall time.