[fix](datetime) Fix the error in date calculation that includes constants (#20863)
before
```
mysql> select hours_add('2023-03-30 22:23:45.23452',8);
+-------------------------------------+
| hours_add('2023-03-30 22:23:45', 8) |
+-------------------------------------+
| 2023-03-31 06:23:45 |
+-------------------------------------+
mysql> select date_add('2023-03-30 22:23:45.23452',8);
+------------------------------------+
| date_add('2023-03-30 22:23:45', 8) |
+------------------------------------+
| 2023-04-07 22:23:45 |
+------------------------------------+
mysql [test]>select hours_add('2023-03-30 22:23:45.23452',8);
+-------------------------------------------+
| hours_add('2023-03-30 22:23:45.23452', 8) |
+-------------------------------------------+
| 2023-03-31 06:23:45.000234 |
+-------------------------------------------+
```
after
```
mysql [test]>select hours_add('2023-03-30 22:23:45.23452',8);
+-------------------------------------------+
| hours_add('2023-03-30 22:23:45.23452', 8) |
+-------------------------------------------+
| 2023-03-31 06:23:45.23452 |
+-------------------------------------------+
1 row in set (0.01 sec)
mysql [test]>select date_add('2023-03-30 22:23:45.23452',8);
+------------------------------------------+
| date_add('2023-03-30 22:23:45.23452', 8) |
+------------------------------------------+
| 2023-04-07 22:23:45.23452 |
+------------------------------------------+
1 row in set (0.00 sec)
mysql [test]>set enable_nereids_planner=true;
Query OK, 0 rows affected (0.00 sec)
mysql [test]>set enable_fallback_to_original_planner=false;
Query OK, 0 rows affected (0.00 sec)
mysql [test]>select hours_add('2023-03-30 22:23:45.23452',8);
+-------------------------------------------+
| hours_add('2023-03-30 22:23:45.23452', 8) |
+-------------------------------------------+
| 2023-03-31 06:23:45.23452 |
+-------------------------------------------+
1 row in set (0.03 sec)
mysql [test]>select date_add('2023-03-30 22:23:45.23452',8);
+------------------------------------------+
| days_add('2023-03-30 22:23:45.23452', 8) |
+------------------------------------------+
| 2023-04-07 22:23:45.23452 |
+------------------------------------------+
1 row in set (0.00 sec)
```