diff --git a/folium/plugins/timeline.py b/folium/plugins/timeline.py
index 19edb5394..047a8dbde 100644
--- a/folium/plugins/timeline.py
+++ b/folium/plugins/timeline.py
@@ -1,4 +1,4 @@
-from typing import Optional, TextIO, Union
+from typing import Optional, Union
 
 import isodate
 import jq
@@ -211,12 +211,12 @@ class Timeline(JSCSSMixin, MacroElement):
 
     def __init__(
         self,
-        data: Union[TextIO, str, dict],
-        duration: str = None,
+        data: Union[dict],
+        duration: Optional[str] = None,
         convert: bool = True,
         get_interval: Optional[JsCode] = None,
-        start: Union[str, int, float] = None,
-        end: Union[str, int, float] = None,
+        start: Optional[Union[str, int, float]] = None,
+        end: Optional[Union[str, int, float]] = None,
         add_last_point: bool = True,
         auto_play: bool = True,
         format_output: Optional[JsCode] = format_output,
@@ -277,11 +277,13 @@ def _convert_to_feature_collection(obj) -> dict:
     return {"type": "FeatureCollection", "features": [obj]}
 
 
-def _convert_from_times(obj: dict, duration: str, add_last_point: bool) -> dict:
+def _convert_from_times(
+    obj: dict, duration: Optional[str], add_last_point: bool
+) -> dict:
     "Converts a GeoJson from the TimeDimension format to the Timeline format"
     obj = _convert_to_feature_collection(obj)
     if duration is not None:
-        duration = int(isodate.parse_duration(duration).total_seconds() * 1000)
+        duration = str(int(isodate.parse_duration(duration).total_seconds() * 1000))
     else:
         # if duration is none, we show until the end of times
         duration = (