diff --git a/conda-requirements.txt b/conda-requirements.txt
index 3af0ec5003..fea36e4546 100644
--- a/conda-requirements.txt
+++ b/conda-requirements.txt
@@ -4,7 +4,7 @@
 # Mandatory dependencies
 biggus>=0.14.0
 cartopy
-matplotlib=1.3.1
+matplotlib
 netcdf4
 numpy
 pyke
diff --git a/lib/iris/fileformats/pp_rules.py b/lib/iris/fileformats/pp_rules.py
index 565fd4ac56..7bc2f25829 100644
--- a/lib/iris/fileformats/pp_rules.py
+++ b/lib/iris/fileformats/pp_rules.py
@@ -483,7 +483,7 @@ def date2hours(t):
         epoch_hours = epoch_hours_unit.date2num(t)
         if t.minute == 0 and t.second == 0:
             epoch_hours = round(epoch_hours)
-        return epoch_hours
+        return float(epoch_hours)
 
     def date2year(t_in):
         return t_in.year
diff --git a/minimal-conda-requirements.txt b/minimal-conda-requirements.txt
index acf5bd6762..b08e009eb2 100644
--- a/minimal-conda-requirements.txt
+++ b/minimal-conda-requirements.txt
@@ -4,7 +4,7 @@
 # Mandatory dependencies
 biggus>=0.14.0
 cartopy
-matplotlib=1.3.1
+matplotlib
 netcdf4
 numpy
 pyke