diff --git a/geomagio/api/xml/XMLFactory.py b/geomagio/api/xml/XMLFactory.py
index 4e6850de9180dd511cf76c27977c763e90f04958..d5a63f710a4f036683dc06e4024043150a75126c 100644
--- a/geomagio/api/xml/XMLFactory.py
+++ b/geomagio/api/xml/XMLFactory.py
@@ -10,19 +10,9 @@ class XMLFactory(TimeseriesFactory):
 
     def __init__(
         self,
-        time_format="iso8601",  # could be "iso8601" or "numeric"
         **kwargs,
     ):
-        """
-        Parameters
-        ----------
-        time_format : {"iso8601", "numeric"}
-            - "iso8601": each time value is an ISO8601 string
-            - "numeric": each time value is numeric (epoch milliseconds)
-
-        """
         super().__init__(**kwargs)
-        self.time_format = time_format
 
     def parse_string(self, data: str, **kwargs):
         """
@@ -67,13 +57,10 @@ class XMLFactory(TimeseriesFactory):
         start_time_str = description.findtext("StartTime", default="")
         sample_period = float(description.findtext("SamplePeriod", default="60000"))
 
-        if self.time_format == "numeric":
-            starttime = UTCDateTime(float(start_time_str) / 1000.0)
-        else:
-            try:
-                starttime = UTCDateTime(start_time_str)
-            except Exception:
-                starttime = UTCDateTime()
+        try:
+            starttime = UTCDateTime(start_time_str)
+        except Exception:
+            starttime = UTCDateTime()
 
         # Parse Data
         data_element = root.find("Data")