A similar problem occurs when adding/substracting years when the
current day does not exist in the target year, i.e. only for
February 29th:

This works as expected:
>>> date.Date('2020-02-29') + date.Interval('+4y')
<Date 2024-02-29.00:00:00.000>

Here you can see the issue:
>>> date.Date('2020-02-29') + date.Interval('+1y')
<Date 2021-03-01.00:00:00.000>
>>> date.Date('2020-02-29') + date.Interval('-1y')
<Date 2019-03-01.00:00:00.000>