date_increment_round() and month crossover

I imported some timestamped date fields using the Migrate module. The timestamps were set to five minutes before the end of the month.

The new Drupal site has date fields with interval of 15 minutes. I noticed that the dates were off and traced the problem to date_increment_round().
It does not account for the rounding into the next month. It passes a nonsense value (32 days) to date_date_set() which outputs an incorrect date.
Example, when the value is 2018-07-31 23:55:55 it is rounded to 2018-09-01 00:00:00.

Included patch accounts for monthly crossover