Growing degree days
Accumulated heat above a base temperature, used to time the next application instead of guessing from the calendar.
Take the day's high and low, average them, subtract a base temperature, and if what's left is positive you add it to a running total. Do that every day and you have a number that tracks how much growing the plant has actually done rather than how many days have gone by on the wall.
The base matters and it isn't universal. A base of 0°C is common for tracking a plant growth regulator on cool-season greens; 10°C is common for warm-season work and for insect models. The base you use has to match the model or the label you're following, and using someone else's base with your target is how a program quietly drifts.
The point of tracking it is that two weeks in May and two weeks in July are not the same two weeks. A regulator that lasts 21 days in cool weather can be finished in 12 during a heat run, and the visible result of getting that wrong is a flush of growth on a Friday afternoon before a weekend of play.
If you spray a regulator on a calendar, you are over-applying in cool weather and under-applying in hot weather, and paying for both.
What you're required to record for a pesticide application, how re-entry intervals should actually be managed on a course open to the public, and how to make the record a by-product of the work.