DEPR: DateOffset args don't end in 's'

doc/source/whatsnew/v1.2.0.rst

@@ -265,6 +265,7 @@ Deprecations
 - Deprecated indexing :class:`DataFrame` rows with datetime-like strings ``df[string]``, use ``df.loc[string]`` instead (:issue:`36179`)
 - Deprecated casting an object-dtype index of ``datetime`` objects to :class:`DatetimeIndex` in the :class:`Series` constructor (:issue:`23598`)
 - Deprecated :meth:`Index.is_all_dates` (:issue:`27744`)
+- Use of Date Offset arguments that do not end with 's' overwrites the offset instead of incrementing it. These arguments are deprecated now and slated for removal in a future version (:issue:`36660`)
 
 .. ---------------------------------------------------------------------------
 

pandas/_libs/tslibs/offsets.pyx

@@ -312,6 +312,17 @@ cdef _determine_offset(kwds):
                                'year', 'month', 'week', 'day', 'weekday',
                                'hour', 'minute', 'second', 'microsecond')
 
+    if any(not k.endswith('s') for k in kwds_no_nanos):
+        if 'weekday' not in kwds_no_nanos:
+            warnings.warn(
+                "Date Offset arguments that do not end with 's' overwrites "
+                "the offset instead of incrementing it. These arguments "
+                "are deprecated now and slated for removal in a future "
+                "version.",
+                FutureWarning,
+                stacklevel=1,
+            )
+
     use_relativedelta = False
     if len(kwds_no_nanos) > 0:
         if any(k in _kwds_use_relativedelta for k in kwds_no_nanos):