diff --git a/lib/matplotlib/dates.py b/lib/matplotlib/dates.py
index 3f152d10abaf..8ba98ee8caa2 100644
--- a/lib/matplotlib/dates.py
+++ b/lib/matplotlib/dates.py
@@ -440,9 +440,7 @@ def julian2num(j):
     float or sequence of floats
         Matplotlib date(s)
     """
-    if np.iterable(j):
-        j = np.asarray(j)
-    return j - JULIAN_OFFSET
+    return np.subtract(j, JULIAN_OFFSET)  # Handles both scalar & nonscalar j.
 
 
 def num2julian(n):
@@ -459,9 +457,7 @@ def num2julian(n):
     float or sequence of floats
         Julian date(s)
     """
-    if np.iterable(n):
-        n = np.asarray(n)
-    return n + JULIAN_OFFSET
+    return np.add(n, JULIAN_OFFSET)  # Handles both scalar & nonscalar j.
 
 
 def num2date(x, tz=None):
diff --git a/lib/matplotlib/testing/jpl_units/EpochConverter.py b/lib/matplotlib/testing/jpl_units/EpochConverter.py
index 40b7c856ad08..0cd9f4e96575 100644
--- a/lib/matplotlib/testing/jpl_units/EpochConverter.py
+++ b/lib/matplotlib/testing/jpl_units/EpochConverter.py
@@ -2,6 +2,7 @@
 
 import numpy as np
 
+from matplotlib import cbook
 import matplotlib.units as units
 import matplotlib.dates as date_ticker
 
@@ -95,28 +96,14 @@ def convert(value, unit, axis):
         # Delay-load due to circular dependencies.
         import matplotlib.testing.jpl_units as U
 
-        isNotEpoch = True
-        isDuration = False
-
-        if np.iterable(value) and not isinstance(value, str):
-            if len(value) == 0:
-                return []
-            else:
-                return [EpochConverter.convert(x, unit, axis) for x in value]
-
-        if isinstance(value, U.Epoch):
-            isNotEpoch = False
-        elif isinstance(value, U.Duration):
-            isDuration = True
-
-        if (isNotEpoch and not isDuration and
-           units.ConversionInterface.is_numlike(value)):
+        if not cbook.is_scalar_or_string(value):
+            return [EpochConverter.convert(x, unit, axis) for x in value]
+        if (units.ConversionInterface.is_numlike(value)
+                and not isinstance(value, (U.Epoch, U.Duration))):
             return value
-
         if unit is None:
             unit = EpochConverter.default_units(value, axis)
-
-        if isDuration:
+        if isinstance(value, U.Duration):
             return EpochConverter.duration2float(value)
         else:
             return EpochConverter.epoch2float(value, unit)
@@ -131,10 +118,7 @@ def default_units(value, axis):
         = RETURN VALUE
         - Returns the default units to use for value.
         """
-        frame = None
-        if np.iterable(value) and not isinstance(value, str):
-            return EpochConverter.default_units(value[0], axis)
+        if cbook.is_scalar_or_string(value):
+            return value.frame()
         else:
-            frame = value.frame()
-
-        return frame
+            return EpochConverter.default_units(value[0], axis)
diff --git a/lib/matplotlib/testing/jpl_units/UnitDblConverter.py b/lib/matplotlib/testing/jpl_units/UnitDblConverter.py
index 352c30b84413..a43a92a892b7 100644
--- a/lib/matplotlib/testing/jpl_units/UnitDblConverter.py
+++ b/lib/matplotlib/testing/jpl_units/UnitDblConverter.py
@@ -2,6 +2,7 @@
 
 import numpy as np
 
+from matplotlib import cbook
 import matplotlib.units as units
 import matplotlib.projections.polar as polar
 
@@ -83,35 +84,21 @@ def convert(value, unit, axis):
         # Delay-load due to circular dependencies.
         import matplotlib.testing.jpl_units as U
 
-        isNotUnitDbl = True
-
-        if np.iterable(value) and not isinstance(value, str):
-            if len(value) == 0:
-                return []
-            else:
-                return [UnitDblConverter.convert(x, unit, axis) for x in value]
-
-        # We need to check to see if the incoming value is actually a
-        # UnitDbl and set a flag.  If we get an empty list, then just
-        # return an empty list.
-        if isinstance(value, U.UnitDbl):
-            isNotUnitDbl = False
-
+        if not cbook.is_scalar_or_string(value):
+            return [UnitDblConverter.convert(x, unit, axis) for x in value]
         # If the incoming value behaves like a number, but is not a UnitDbl,
         # then just return it because we don't know how to convert it
         # (or it is already converted)
-        if isNotUnitDbl and units.ConversionInterface.is_numlike(value):
+        if (units.ConversionInterface.is_numlike(value)
+                and not isinstance(value, U.UnitDbl)):
             return value
-
         # If no units were specified, then get the default units to use.
         if unit is None:
             unit = UnitDblConverter.default_units(value, axis)
-
         # Convert the incoming UnitDbl value/values to float/floats
         if isinstance(axis.axes, polar.PolarAxes) and value.type() == "angle":
             # Guarantee that units are radians for polar plots.
             return value.convert("rad")
-
         return value.convert(unit)
 
     @staticmethod
@@ -125,10 +112,9 @@ def default_units(value, axis):
         - Returns the default units to use for value.
         Return the default unit for value, or None.
         """
-
         # Determine the default units based on the user preferences set for
         # default units when printing a UnitDbl.
-        if np.iterable(value) and not isinstance(value, str):
-            return UnitDblConverter.default_units(value[0], axis)
-        else:
+        if cbook.is_scalar_or_string(value):
             return UnitDblConverter.defaults[value.type()]
+        else:
+            return UnitDblConverter.default_units(value[0], axis)