diff --git a/src/stepcount/stepcount.py b/src/stepcount/stepcount.py
index 5bfd208..1ab6542 100644
--- a/src/stepcount/stepcount.py
+++ b/src/stepcount/stepcount.py
@@ -212,7 +212,7 @@ def _mean(x):
 def summarize_steps(Y, steptol=3, adjust_estimates=False):
     """ Summarize step count data """
 
-    dt = pd.Timedelta(infer_freq(Y.index)).seconds
+    dt = infer_freq(Y.index).total_seconds()
     W = Y.mask(~Y.isna(), Y >= steptol).astype('float')
 
     if adjust_estimates:
@@ -224,7 +224,10 @@ def summarize_steps(Y, steptol=3, adjust_estimates=False):
         skipna = True
 
     def _sum(x):
-        if not skipna and x.isna().any():
+        na = x.isna()
+        if not skipna and na.any():
+            return np.nan
+        if na.all():  # have to do this explicitly because pandas' .sum() returns 0 if all-NaN
             return np.nan
         return x.sum()
 
@@ -392,7 +395,7 @@ def na_to_median(x):
             .transform(na_to_median)
         )
 
-    dt = pd.Timedelta(infer_freq(Y.index)).seconds
+    dt = infer_freq(Y.index).total_seconds()
     steptol_in_minutes = steptol * 60 / dt  # rescale steptol to steps/min
     minutely = Y.resample('T').sum().rename('Steps')  # steps/min