The easiest way to track late drop-offs would be to select a Visit Type of Late, and to record both the correct Scheduled Time and the Actual Time values. (You can also create a custom label for the Visit Type drop down list if you want to.)
If the time is recorded correctly, the Tracker will calculate the time difference and will flag the Actual Hours cell for that day in a light red, indicating there was a time shortage. You can set one of the Event Flags to help visually note these days in the Tracker Month view.
We also recommend putting a note in one of the text boxes stating that the dropoff was late. Some people put a text tag in the note such as "LDO" to help in future searches for these events.