Log it the day you do it
The single most common way students lose service hours is reconstructing them in April from memory. Nobody remembers whether the food-bank shift in November was three hours or four, and a coordinator asked to verify a year-old estimate will round down.
Two minutes on the day, with the date and what you actually did, is worth more than an afternoon of archaeology in the spring. That is the whole reason this exists as a tracker rather than a calculator.