diff --git a/docs/sphinx/source/whatsnew/v0.7.2.rst b/docs/sphinx/source/whatsnew/v0.7.2.rst
index 2eda805c1e..d49afad155 100644
--- a/docs/sphinx/source/whatsnew/v0.7.2.rst
+++ b/docs/sphinx/source/whatsnew/v0.7.2.rst
@@ -45,6 +45,8 @@ Bug fixes
   and various test functions.
 * Fix :py:func:`~pvlib.iotools.read_tmy3` so that when coerced to a single year
   the TMY3 index will be monotonically increasing. (:pull:`910`)
+* Fix :py:func:`pvlib.spa.julian_day_dt` so that microseconds are scaled
+  correctly (:issue:`940`) (:pull:`942`)
 
 Testing
 ~~~~~~~
diff --git a/pvlib/spa.py b/pvlib/spa.py
index 6c99a03f0c..743df5ce8e 100644
--- a/pvlib/spa.py
+++ b/pvlib/spa.py
@@ -430,7 +430,7 @@ def julian_day_dt(year, month, day, hour, minute, second, microsecond):
         month = month+12
     a = int(year/100)
     b = 2 - a + int(a * 0.25)
-    frac_of_day = (microsecond + (second + minute * 60 + hour * 3600)
+    frac_of_day = (microsecond / 1e6 + (second + minute * 60 + hour * 3600)
                    ) * 1.0 / (3600*24)
     d = day + frac_of_day
     jd = (int(365.25 * (year + 4716)) + int(30.6001 * (month + 1)) + d +
diff --git a/pvlib/tests/test_spa.py b/pvlib/tests/test_spa.py
index 196acd72cb..70b3b9090d 100644
--- a/pvlib/tests/test_spa.py
+++ b/pvlib/tests/test_spa.py
@@ -98,7 +98,8 @@
 class SpaBase(object):
     """Test functions common to numpy and numba spa"""
     def test_julian_day_dt(self):
-        dt = times.tz_convert('UTC')[0]
+        # add 1us manually to the test timestamp (GH #940)
+        dt = times.tz_convert('UTC')[0] + pd.Timedelta(1, unit='us')
         year = dt.year
         month = dt.month
         day = dt.day
@@ -106,7 +107,7 @@ def test_julian_day_dt(self):
         minute = dt.minute
         second = dt.second
         microsecond = dt.microsecond
-        assert_almost_equal(JD,
+        assert_almost_equal(JD + 1e-6 / (3600*24),  # modify expected JD by 1us
                             self.spa.julian_day_dt(
                                 year, month, day, hour,
                                 minute, second, microsecond), 6)