diff --git a/docs/source/whatsnew.rst b/docs/source/whatsnew.rst
index 16ce46c..d2c7eeb 100644
--- a/docs/source/whatsnew.rst
+++ b/docs/source/whatsnew.rst
@@ -14,6 +14,8 @@ Changed
 ^^^^^^^
 * matplotlib is now an optional ``doc`` dependency instead of a required
   dependency. (:pull:`146`)
+* Add support for correctly handling sub-second timestamp resolution in
+  :py:func:`solposx.solarposition.usno`. (:issue:`156`, :pull:`157`)
 
 Added
 ^^^^^
diff --git a/src/solposx/solarposition/usno.py b/src/solposx/solarposition/usno.py
index c730ed3..3628da5 100644
--- a/src/solposx/solarposition/usno.py
+++ b/src/solposx/solarposition/usno.py
@@ -85,9 +85,7 @@ def usno(times, latitude, longitude, *, delta_t=67.0, gmst_option=1):
     d = asind(sind(e) * sind(L))
 
     # JD_0 is the Julian date of the previous midnight (0h) UT1
-    midnight = pd.DatetimeIndex(
-        [t.replace(hour=0, minute=0, second=0) for t in times_utc]
-    )
+    midnight = times_utc.normalize()  # Convert times_utc to midnight
 
     JD_0 = midnight.to_julian_date()