Jamieson, Fausset and Brown on Ezekiel 33:21

21. twelfth year ... tenth month--a year and a half after the capture of the city (Jr 39:2; 52:5, 6), in the eleventh year and fourth month. The one who escaped (as foretold, Eze 24:26) may have been so long on the road through fear of entering the enemy's country [Henderson]; or, the singular is used for the plural in a collective sense, "the escaped remnant." Compare similar phrases, "the escaped of Moab," Is 15:9; "He that escapeth of them," Am 9:1. Naturally the reopening of the prophet's mouth for consolation would be deferred till the number of the escaped remnant was complete: the removal of such a large number would easily have occupied seventeen or eighteen months.