Issue #2456521 by pjonckiere, mpdonadio, jhodgdon, rteijeiro, David_Rothstein,...

Issue #2456521 by pjonckiere, mpdonadio, jhodgdon, rteijeiro, David_Rothstein, xjm: Add DateFormatter::formatDiff() as a non-buggy alternative to DateFormatter::formatInterval() when the start and end of the interval are known