diff --git a/geomagio/Controller.py b/geomagio/Controller.py
index 458e74e43c7b72525ab4a996949837260068109a..70af14041ab4a2862515b39c75a5f58da80af8ff 100644
--- a/geomagio/Controller.py
+++ b/geomagio/Controller.py
@@ -653,11 +653,11 @@ def get_previous_interval(
     Returns
     -------
     Previous interval of approximately the same size.
-    Interval is rounded to nearest second, and ends one microsecond earlier.
+    Interval is rounded to nearest second, and ends one millisecond earlier.
     """
     # round to nearest second to recover removed microsecond from repeated calls
     interval_size = round(end - start)
-    return (start - interval_size, start - 1e-6)
+    return (start - interval_size, start - 1e-3)
 
 
 def get_realtime_interval(interval_seconds: int) -> Tuple[UTCDateTime, UTCDateTime]:
diff --git a/test/Controller_test.py b/test/Controller_test.py
index 9acab443707206462f07ff38c9d7e14efd285dbd..bff8a8fdb44e354403f9940d8e07974f9b2369d0 100644
--- a/test/Controller_test.py
+++ b/test/Controller_test.py
@@ -301,12 +301,12 @@ def test_get_previous_interval():
     previous = get_previous_interval(start=start, end=end)
     assert previous == (
         UTCDateTime("2022-01-04T23:00:00"),
-        UTCDateTime("2022-01-04T23:59:59.999999Z"),
+        UTCDateTime("2022-01-04T23:59:59.999Z"),
     )
     # previous interval still starts at beginning of previous hour
-    # even though interval is one microsecond smaller
+    # even though interval is one millisecond smaller
     previous = get_previous_interval(*previous)
     assert previous == (
         UTCDateTime("2022-01-04T22:00:00"),
-        UTCDateTime("2022-01-04T22:59:59.999999Z"),
+        UTCDateTime("2022-01-04T22:59:59.999Z"),
     )