Late arrival explanation
Arriving late is a small event that becomes a large one only when nobody writes it down. Three or four lines, filed the same morning, keeps it small.
Questions people ask about this letter
Does a made-up hour cancel the late mark?
Usually not. The record shows both: you started late and you worked the hours. Those are separate facts and merging them loses one.
How many notes before it becomes a formal matter?
Whatever your policy says, applied the same way to everyone. An unwritten threshold is applied differently by every supervisor who holds it.
Is traffic an acceptable cause?
Once, obviously. Weekly, it is a commute that does not fit the shift, and the honest fix is a shift change request rather than a monthly note.