diff --git a/bali/db/comparator.py b/bali/db/comparators.py
similarity index 100%
rename from bali/db/comparator.py
rename to bali/db/comparators.py
diff --git a/bali/db/connection.pyi b/bali/db/connection.pyi
index fcc9a47..176b678 100644
--- a/bali/db/connection.pyi
+++ b/bali/db/connection.pyi
@@ -18,11 +18,11 @@ class BaseModel:
     updated_time: _DateTimeField
     is_active: _BooleanField
 
-    def _asdict(self, include_hybrid_properties=__asdict_include_hybrid_properties__) -> Dict[str, Any]: ...
+    def _asdict(self, **kwargs) -> Dict[str, Any]: ...
 
-    def to_dict(self, include_hybrid_properties=__asdict_include_hybrid_properties__) -> Dict[str, Any]: ...
+    def to_dict(self, **kwargs) -> Dict[str, Any]: ...
 
-    def dict(self, include_hybrid_properties=__asdict_include_hybrid_properties__) -> Dict[str, Any]: ...
+    def dict(self, **kwargs) -> Dict[str, Any]: ...
 
     @classmethod
     def exists(cls: Type[_M], **attrs) -> bool: ...
diff --git a/bali/db/models.py b/bali/db/models.py
index 8ef549f..6b54218 100644
--- a/bali/db/models.py
+++ b/bali/db/models.py
@@ -99,7 +99,12 @@ def delete(self):
             db.session.delete(self)
             db.session.commit() if context_auto_commit.get() else db.session.flush()
 
-        def _asdict(self, include_hybrid_properties=__asdict_include_hybrid_properties__):
+        def _asdict(self, **kwargs):
+            include_hybrid_properties = kwargs.setdefault(
+                "include_hybrid_properties",
+                self.__asdict_include_hybrid_properties__
+            )
+
             output_fields = []
             for i in inspect(type(self)).all_orm_descriptors:
                 if isinstance(i, InstrumentedAttribute):
diff --git a/bali/utils/timezone.py b/bali/utils/timezone.py
index 855118b..24477f2 100644
--- a/bali/utils/timezone.py
+++ b/bali/utils/timezone.py
@@ -1,5 +1,6 @@
+import calendar
 import os
-from datetime import datetime
+from datetime import datetime, date, timedelta
 from typing import Union
 
 import pytz
@@ -66,7 +67,7 @@ def make_naive(
     return value.astimezone(timezone).replace(tzinfo=None)
 
 
-def localtime(value: datetime = None, timezone: StrTzInfoType = None):
+def localtime(value: datetime = None, timezone: StrTzInfoType = None) -> datetime:
     value, timezone = value or now(), timezone or get_current_timezone()
     if isinstance(timezone, str):
         timezone = pytz.timezone(timezone)
@@ -75,5 +76,26 @@ def localtime(value: datetime = None, timezone: StrTzInfoType = None):
     return value.astimezone(timezone)
 
 
-def localdate(value: datetime = None, timezone: StrTzInfoType = None):
+def localdate(value: datetime = None, timezone: StrTzInfoType = None) -> date:
     return localtime(value, timezone).date()
+
+
+def start_of(
+        granularity: str,
+        value: datetime = None,
+        *,
+        timezone: StrTzInfoType = None,
+) -> datetime:
+    value = localtime(value, timezone)
+    if granularity == "year":
+        value = value.replace(month=1, day=1)
+    elif granularity == "month":
+        value = value.replace(day=1)
+    elif granularity == "week":
+        value = value - timedelta(days=calendar.weekday(value.year, value.month, value.day))
+    elif granularity == "day":
+        pass
+    else:
+        raise ValueError("Granularity must be year, month, week or day")
+
+    return value.replace(hour=0, minute=0, second=0, microsecond=0)