Add clarifying comments to Date testing

mpas_analysis/test/test_date.py

@@ -129,6 +129,10 @@ def test_date(self):
         timedelta2 = datetime.timedelta(days=20)
         self.assertEqual(timedelta1, timedelta2)
 
+        # since pandas and xarray use the numpy type 'datetime[ns]`, which
+        # has a limited range of dates, the date 0001-01-01 gets increased to
+        # the minimum allowed year boundary, 1678-01-01 to avoid invalid
+        # dates.
         date = Date(dateString='0001-01-01', isInterval=False)
         datetime1 = date.to_datetime(yearOffset=0)
         datetime2 = datetime.datetime(year=1678, month=1, day=1)
@@ -139,6 +143,10 @@ def test_date(self):
         datetime2 = datetime.datetime(year=1850, month=1, day=1)
         self.assertEqual(datetime1, datetime2)
 
+        # since pandas and xarray use the numpy type 'datetime[ns]`, which
+        # has a limited range of dates, the date 9999-01-01 gets decreased to
+        # the maximum allowed year boundary, 2262-01-01 to avoid invalid
+        # dates.
         date = Date(dateString='9999-01-01', isInterval=False)
         datetime1 = date.to_datetime(yearOffset=0)
         datetime2 = datetime.datetime(year=2262, month=1, day=1)