setMinDate(...) for DatePicker doesn't work when invoked a second time

This happens because method setMinDate() has check

 if (mTempDate.get(Calendar.YEAR) == mMinDate.get(Calendar.YEAR)
                && mTempDate.get(Calendar.DAY_OF_YEAR) != mMinDate.get(Calendar.DAY_OF_YEAR){
            return;
 }

Simple workaround is to set min date with different year at first, for example

mPicker.setMinDate(0);

mPicker.setMinDate(new LocalDate().minusWeeks(2)
                                .toDateTimeAtStartOfDay().getMillis());

It works for me.

As said above, you can bypass the check by calling those before actually changing the value:

setMinDate(0);
setMaxDate(Long.MAX_VALUE);

If you want to reset the minimum or maximum value to its default, you can use the following values:

setMinDate(-2208902400000L);  // Jan 1, 1900
setMaxDate(4102531200000L);  // Jan 1, 2100