Last third of the night calculator
Divide the night between Maghrib and Fajr into three, and see when the last third starts and ends for your location and date.
How the night is divided
The night is taken as the span from Maghrib to the following morning’s Fajr, and split into three equal parts. The last of those three is what people usually mean by the last third of the night.
Note what the night is not: it is not sunset to sunrise. Fajr arrives well before the sun does, often an hour and a half earlier, so measuring to sunrise would make the night longer than it is and push every boundary later. At a British latitude in winter that mistake moves the last third by more than an hour.
Which night the date refers to
A night belongs to the evening it starts in. Choosing the 21st gives you the night that begins at Maghrib on the 21st and ends at Fajr on the 22nd, so the calculation looks at both days and takes the later Fajr. It is a small difference, Fajr shifts by around a minute a day, but it is the difference between measuring this night and the last one.
How much it moves through the year
A great deal, at higher latitudes. In London a night in December runs about fourteen hours, so each third is nearly five and the final one starts a little after one in the morning. In June the whole night is short enough that each third is under two hours. Nearer the equator the figure barely changes at all.
Where the calculation stops
If either end of the night is missing, no Maghrib, or no Fajr. There is no span to divide, and the tool reports that rather than estimating. That happens on summer dates above roughly 48 degrees of latitude. The conventions communities use in those places are offered as a setting, and whichever you choose will move the boundaries with it.