Someone who knows better can correct me if this is wrong.
I believe it is now a rule that timing mats should be placed BEFORE the start line and AFTER the finish line. This is intended to assure that the recorded time is not less than that actually run. This, as in other aspects of course layout, is done on behalf of the records-keepers. Some think this is overkill, and many races ignore it, either through ignorance or intention.
In any case, if the mats are laid out as above, there will be a problem when runners cross the start/finish line from the same direction. The mats will need to be moved from one side of the line to the other.