diff --git a/geomagio/pydantic_utcdatetime.py b/geomagio/pydantic_utcdatetime.py
index 31a2bb39b79ffe6c45d5ec4d81508f8d0b16bf14..52ce67e262cf5d10c9698e88583e1089cdadd68e 100644
--- a/geomagio/pydantic_utcdatetime.py
+++ b/geomagio/pydantic_utcdatetime.py
@@ -42,7 +42,7 @@ class UTCDateTimeError(PydanticValueError):
 
 
 def format_utcdatetime(o: UTCDateTime) -> str:
-    return o.isoformat()
+    return o.strftime("%Y-%m-%dT%H:%M:%S.%fZ")
 
 
 def parse_utcdatetime(