Performance improvement in :meth:Series.to_excel and :meth:DataFrame.to_excel (:class:ExcelFormatter): processing dates can be up to 4% faster. (related to :issue:44764)

Author: Sylvain MARIE

doc/source/whatsnew/v1.5.0.rst

@@ -387,6 +387,7 @@ Performance improvements
 - Performance improvement in :class:`DataFrame` and :class:`Series` constructors for extension dtype scalars (:issue:`45854`)
 - Performance improvement in :class:`BusinessHour`, ``str`` and ``repr`` are now 4 times faster ! (related to :issue:`44764`)
 - Performance improvement in :class:`DatetimeArray` and :class:`DatetimeIndex`: string formatting is now up to 80% faster (as fast as default) when one of the default strftime formats ``"%Y-%m-%d %H:%M:%S"`` or ``"%Y-%m-%d %H:%M:%S.%f"`` is used. (:issue:`44764`)
+- Performance improvement in :meth:`Series.to_excel` and :meth:`DataFrame.to_excel` (:class:`ExcelFormatter`): processing dates can be up to 4% faster. (related to :issue:`44764`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_150.bug_fixes:

pandas/io/excel/_odswriter.py

@@ -189,15 +189,15 @@ def _make_table_cell(self, cell) -> tuple[object, Any]:
             value = str(val).lower()
             pvalue = str(val).upper()
         if isinstance(val, datetime.datetime):
-            value = val.isoformat()
-            pvalue = val.strftime("%c")
+            value = val.isoformat()  # fast formatting
+            pvalue = val.strftime("%c")  # slow but locale-dependent
             return (
                 pvalue,
                 TableCell(valuetype="date", datevalue=value, attributes=attributes),
             )
         elif isinstance(val, datetime.date):
-            value = val.strftime("%Y-%m-%d")
-            pvalue = val.strftime("%x")
+            value = f"{val.year}-{val.month:02d}-{val.day:02d}"  # fast formatting
+            pvalue = val.strftime("%x")  # slow but locale-dependent
             return (
                 pvalue,
                 TableCell(valuetype="date", datevalue=value, attributes=attributes),