diff --git a/sqlalchemy_bigquery/base.py b/sqlalchemy_bigquery/base.py
index 0204bc92..d2c75580 100644
--- a/sqlalchemy_bigquery/base.py
+++ b/sqlalchemy_bigquery/base.py
@@ -832,61 +832,81 @@ def _process_time_partitioning(
         function returns:
         "PARTITION BY TIMESTAMP_TRUNC(event_timestamp, DAY)".
 
-        Current inputs allowed by BQ and covered by this function include:
+        Current inputs allowed by BQ AND covered by this function include:
         * _PARTITIONDATE
         * DATETIME_TRUNC(<datetime_column>, DAY/HOUR/MONTH/YEAR)
         * TIMESTAMP_TRUNC(<timestamp_column>, DAY/HOUR/MONTH/YEAR)
         * DATE_TRUNC(<date_column>, MONTH/YEAR)
-
-        Additional options allowed by BQ but not explicitly covered by this
-        function include:
         * DATE(_PARTITIONTIME)
         * DATE(<timestamp_column>)
         * DATE(<datetime_column>)
         * DATE column
         """
 
-        sqltypes = {
-            "_PARTITIONDATE": ("_PARTITIONDATE", None),
-            "TIMESTAMP": ("TIMESTAMP_TRUNC", {"DAY", "HOUR", "MONTH", "YEAR"}),
-            "DATETIME": ("DATETIME_TRUNC", {"DAY", "HOUR", "MONTH", "YEAR"}),
-            "DATE": ("DATE_TRUNC", {"MONTH", "YEAR"}),
-        }
-
-        # Extract field (i.e <column_name> or _PARTITIONDATE)
-        # AND extract the name of the column_type (i.e. "TIMESTAMP", "DATE",
+        # Extract field if given (i.e <column_name>) or _PARTITIONDATE if not given
+        # AND extract the name of the column_type (i.e. is it a "TIMESTAMP", "DATE",
         # "DATETIME", "_PARTITIONDATE")
+        # Also extract the time_partitioning.type_ (i.e. the truncation granularity:
+        # HOUR, DAY, MONTH, YEAR)
         if time_partitioning.field is not None:
             field = time_partitioning.field
             column_type = table.columns[field].type.__visit_name__.upper()
 
+            # Extract time_partitioning.type_ (DAY, HOUR, MONTH, YEAR)
+            # i.e. generates one partition per type (1/DAY, 1/HOUR)
+            # NOTE: if time_partitioning.type_ == None, the python-bigquery library
+            # will eventually overwrite it with a default of DAY.
+            partitioning_period = time_partitioning.type_
+
         else:
+            # If no field is given, default to "_PARTITIONDATE" as the 
+            # field to partition on. In addition, to normalize the processing in
+            # the remainder of this function, set column_type and partitioning_period
+            # as shown below.
             field = "_PARTITIONDATE"
             column_type = "_PARTITIONDATE"
+            partitioning_period = None
+
+        # TODO: move this dictionary outside the function or to top of function
+        sqltypes_w_no_partitioning_period = {
+            # Keys are columns, values are functions
+            "_PARTITIONDATE": None,
+            "_PARTITIONTIME": "DATE", # <date function>
+            "DATE": None, # 'DATE' is a <date_column> not a function
+            "DATETIME": "DATE", # <date function>
+            "TIMESTAMP": "DATE", # <date function>
+        }
 
-        # Extract time_partitioning.type_ (DAY, HOUR, MONTH, YEAR)
-        # i.e. generates one partition per type (1/DAY, 1/HOUR)
-        # NOTE: if time_partitioning.type_ == None, it gets
-        # immediately overwritten by python-bigquery to a default of DAY.
-        partitioning_period = time_partitioning.type_
+        # TODO: move this dictionary outside the function or to top of function
+        sqltypes_w_partitioning_period = {
+            # Keys are columns, values are (functions, {allowed_partioning_periods})
+            #"_PARTITIONDATE": ("_PARTITIONDATE", {}),
+            "DATE": ("DATE_TRUNC", {"MONTH", "YEAR"}),
+            "DATETIME": ("DATETIME_TRUNC", {"DAY", "HOUR", "MONTH", "YEAR"}),
+            "TIMESTAMP": ("TIMESTAMP_TRUNC", {"DAY", "HOUR", "MONTH", "YEAR"}),
+        }
 
-        # Extract the truncation_function (i.e. DATE_TRUNC)
-        # and the set of allowable partition_periods
+        # Extract truncation_function (i.e. DATE_TRUNC()) or a default value if
+        # a truncation_function is not used (i.e. for _PARTITIONDATE)
+        # Also extract the set of allowable partition_periods
         # that can be used in that function
-        trunc_fn, allowed_partitions = sqltypes[column_type]
+        if partitioning_period is None:
+            function = sqltypes_w_no_partitioning_period[column_type]
+        else:
+            function, allowed_partitions = sqltypes_w_partitioning_period[column_type]            
 
         # Create output:
-        # Special Case: _PARTITIONDATE does NOT use a function or partitioning_period
-        if trunc_fn == "_PARTITIONDATE":
+        # Special Case 1: _PARTITIONDATE does NOT use a function or partitioning_period
+        if function is None:
             return f"PARTITION BY {field}"
 
-        # Special Case: BigQuery will not accept DAY as partitioning_period for
+        # Special Case 2: BigQuery will not accept DAY as partitioning_period for
         # DATE_TRUNC.
-        # However, the default argument in python-bigquery for TimePartioning
+        # However, the default argument in python-bigquery for TimePartitioning
         # is DAY. This case overwrites that to avoid making a breaking change in
         # python-bigquery.
         # https://github.com/googleapis/python-bigquery/blob/a4d9534a900f13ae7355904cda05097d781f27e3/google/cloud/bigquery/table.py#L2916
-        if trunc_fn == "DATE_TRUNC" and partitioning_period == "DAY":
+        if function == "DATE_TRUNC" and partitioning_period == "DAY":
             raise ValueError(
                 "The TimePartitioning.type_ must be one of: "
                 f"{allowed_partitions}, received {partitioning_period}."
@@ -902,7 +922,7 @@ def _process_time_partitioning(
                 f"{allowed_partitions}, received {partitioning_period}."
             )
 
-        return f"PARTITION BY {trunc_fn}({field}, {partitioning_period})"
+        return f"PARTITION BY {function}({field}, {partitioning_period})"
 
     def _process_range_partitioning(
         self, table: Table, range_partitioning: RangePartitioning