remove dtype from encoding for datetime64/timedelta64 variables to prevent unnecessary casts

Author: kmuehlbauer

xarray/coding/times.py

@@ -777,6 +777,11 @@ def encode(self, variable: Variable, name: T_Name = None) -> Variable:
             safe_setitem(attrs, "units", units, name=name)
             safe_setitem(attrs, "calendar", calendar, name=name)
 
+            # remove dtype from encoding to prevent unnecessary casts
+            # see GH #1064
+            if "dtype" in encoding:
+                encoding.pop("dtype")
+
             return Variable(dims, data, attrs, encoding, fastpath=True)
         else:
             return variable
@@ -810,6 +815,11 @@ def encode(self, variable: Variable, name: T_Name = None) -> Variable:
             data, units = encode_cf_timedelta(data, encoding.pop("units", None))
             safe_setitem(attrs, "units", units, name=name)
 
+            # remove dtype from encoding to prevent unnecessary casts
+            # see GH #1064
+            if "dtype" in encoding:
+                encoding.pop("dtype")
+
             return Variable(dims, data, attrs, encoding, fastpath=True)
         else:
             return variable