Test fix for 31st of month corner case

On the last day of a 31-day-long month that precedes a 30-day-long month (like today, the 31st of March), `DateTime >> 1 >> 1` is not equal to `DateTime >> 2`.

In the case of `DateTime >> 1 >> 1`, March 31 becomes April 30, which then becomes May 30, but in the case of `DateTime >> 2`, March 31 becomes May 31.

In the test below, I incorrectly assumed `DateTime >> 1 >> 1` would always equal `DateTime >> 2`. This has now been fixed.