There is a finite number of days and weeks between the two dates.

You didn't ask to the number of days and/or weeks between two dates. As I already mentioned, that would be $endDate->delta_days($dt1).

So, forget about the month(s), why are the weeks and days incorrect?

huh? They aren't. There is 1 month, 10 days, 17 hours and 1 minute between 2015-03-19 17:01 and 2015-02-30 00:00, and the output said 10 days.