gh-102450: Add ISO-8601 alternative for midnight to fromisoformat() calls. (#105856)

* Add NEWS.d entry

* Allow ISO-8601 24:00 alternative to midnight on datetime.time.fromisoformat()

* Allow ISO-8601 24:00 alternative to midnight on datetime.datetime.fromisoformat()

* Add NEWS.d entry

* Improve error message when hour is 24 and minute/second/microsecond is not 0

* Add tests for 24:00 fromisoformat

* Remove duplicate call to days_in_month() by storing in variable

* Add Python implementation

* Fix Lint

* Fix differing error msg in datetime.fromisoformat implementations when 24hrs has non-zero time component(s)

* Fix using time components inside tzinfo in Python implementation

* Don't parse tzinfo in C implementation when invalid iso midnight

* Remove duplicated variable in datetime test assertion line

* Add self to acknowledgements

* Remove duplicate NEWS entry

* Linting

* Add missing test case for when wrapping the year makes it invalid (too large)

Modules/_datetimemodule.c

@@ -4997,6 +4997,14 @@ time_fromisoformat(PyObject *cls, PyObject *tstr) {
         goto invalid_string_error;
     }
 
+    if (hour == 24) {
+        if (minute == 0 && second == 0 && microsecond == 0) {
+            hour = 0;
+        } else {
+            goto invalid_iso_midnight;
+        }
+    }
+
     PyObject *tzinfo = tzinfo_from_isoformat_results(rv, tzoffset,
                                                      tzimicrosecond);
 
@@ -5015,6 +5023,10 @@ time_fromisoformat(PyObject *cls, PyObject *tstr) {
     Py_DECREF(tzinfo);
     return t;
 
+invalid_iso_midnight:
+    PyErr_SetString(PyExc_ValueError, "minute, second, and microsecond must be 0 when hour is 24");
+    return NULL;
+
 invalid_string_error:
     PyErr_Format(PyExc_ValueError, "Invalid isoformat string: %R", tstr);
     return NULL;
@@ -5861,6 +5873,26 @@ datetime_fromisoformat(PyObject *cls, PyObject *dtstr)
         goto error;
     }
 
+    if ((hour == 24) && (month <= 12))  {
+        int d_in_month = days_in_month(year, month);
+        if (day <= d_in_month) {
+            if (minute == 0 && second == 0 && microsecond == 0) {
+                // Calculate midnight of the next day
+                hour = 0;
+                day += 1;
+                if (day > d_in_month) {
+                    day = 1;
+                    month += 1;
+                    if (month > 12) {
+                        month = 1;
+                        year += 1;
+                    }
+                }
+            } else {
+                goto invalid_iso_midnight;
+            }
+        }
+    }
     PyObject *dt = new_datetime_subclass_ex(year, month, day, hour, minute,
                                             second, microsecond, tzinfo, cls);
 
@@ -5868,6 +5900,10 @@ datetime_fromisoformat(PyObject *cls, PyObject *dtstr)
     Py_DECREF(dtstr_clean);
     return dt;
 
+invalid_iso_midnight:
+    PyErr_SetString(PyExc_ValueError, "minute, second, and microsecond must be 0 when hour is 24");
+    return NULL;
+
 invalid_string_error:
     PyErr_Format(PyExc_ValueError, "Invalid isoformat string: %R", dtstr);