UT_COUNTDOWN — Remaining time (function)

Works with any timer: the standard TON/TOF/TP, or UT_TONR. Typical use is
showing "time left" on an HMI.

Pins

	Pin	Type	Default	What to wire
in	`PT`	TIME	T#0s	Timer preset (`MyTon.PT`)
in	`ET`	TIME	T#0s	Timer elapsed (`MyTon.ET`)
returns		TIME		`PT − ET`, never negative

Remaining := UT_COUNTDOWN(PT := Delay.PT, ET := Delay.ET);

ET at or past PT gives 0.
A PT of 0 or less gives 0. A negative ET counts as 0, so the result is never more than PT.
Show it on an HMI with UT_TIME_SPLIT for days, hours, minutes and seconds.