diff --git a/weather_mv/loader_pipeline/ee.py b/weather_mv/loader_pipeline/ee.py
index 69b5b637..aecec338 100644
--- a/weather_mv/loader_pipeline/ee.py
+++ b/weather_mv/loader_pipeline/ee.py
@@ -182,15 +182,15 @@ class AssetData:
         name: The EE-safe name of the asset.
         target_path: The location of the asset in GCS.
         channel_names: A list of channel names in the asset.
-        start_time: Image start time in floating point seconds since epoch.
-        end_time: Image end time in floating point seconds since epoch.
+        start_time: Image start time in '%Y-%m-%dT%H:%M:%SZ' format.
+        end_time: Image end time in '%Y-%m-%dT%H:%M:%SZ' format.
         properties: A dictionary of asset metadata.
     """
     name: str
     target_path: str
     channel_names: t.List[str]
-    start_time: float
-    end_time: float
+    start_time: str
+    end_time: str
     properties: t.Dict[str, t.Union[str, float, int]]
 
 
diff --git a/weather_mv/loader_pipeline/sinks.py b/weather_mv/loader_pipeline/sinks.py
index f5e7028e..e77f921e 100644
--- a/weather_mv/loader_pipeline/sinks.py
+++ b/weather_mv/loader_pipeline/sinks.py
@@ -181,8 +181,8 @@ def _get_band_data(i):
             end_time = match_datetime(uri, forecast_time_regex)
         except Exception:
             raise RuntimeError("Wrong regex passed in --forecast_time_regex.")
-        ds.attrs['start_time'] = start_time
-        ds.attrs['end_time'] = end_time
+        ds.attrs['start_time'] = _to_utc_timestring(start_time)
+        ds.attrs['end_time'] = _to_utc_timestring(end_time)
 
     # TODO(#159): Explore ways to capture required metadata using xarray.
     with rasterio.open(filename) as f:
@@ -199,9 +199,11 @@ def _get_band_data(i):
     return ds
 
 
-def _to_utc_timestring(np_time: np.datetime64) -> str:
+def _to_utc_timestring(time: t.Union[np.datetime64, datetime.datetime]) -> str:
     """Turn a numpy datetime64 into UTC timestring."""
-    timestamp = float((np_time - np.datetime64(0, 's')) / np.timedelta64(1, 's'))
+    if isinstance(time, datetime.datetime):
+        return time.strftime('%Y-%m-%dT%H:%M:%SZ')
+    timestamp = float((time - np.datetime64(0, 's')) / np.timedelta64(1, 's'))
     return datetime.datetime.utcfromtimestamp(timestamp).strftime('%Y-%m-%dT%H:%M:%SZ')