The algorithm for finding the IntervalType for a specific scale is implemented in QwtDateScaleDraw::intervalType(). Then the format for the chosen IntervalType is used for converting the tick labels to a string.

So yes, if other interval types might happen in your application ( and your are unhappy with the default formats ) you would have to modify all of them. But of course a format including milliseconds doesn't make too much sense, when all tick labels are aligned to hours as you waste space for appending "000".

Uwe