I'd prefer day-Calendar.Month*3, I think, but that might be mostly for being tainted by the conception that Calendar.Month() is a timerange starting 00:00:00 on the first of the present month, and ending just before that time the next month. In other words, a timerange object like any other that just happens to coincide with present month, whose length depends on when you instantiate it.
Maybe it is in poor taste to consider the type Calendar.Month a good candidate for doing integer multiplication on, though.