alter table modify column to a time type may generate incorrect value

[lcwangchao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

My local time zone is Asia/Shanghai

> create table t(t int);
> insert into t values(NULL);
> set @@time_zone='UTC' --- set a different time zone with local
> alter table t modify column t timestamp not null;
> select now(),t from t;

2. What did you expect to see? (Required)

The value of now() and t should not differs too much.

3. What did you see instead (Required)

These two columns differs two hours

> select t, now() from t
+---------------------+---------------------+
| t                   | now()               |
+---------------------+---------------------+
| 2024-09-12 04:09:36 | 2024-09-12 12:13:47 |
+---------------------+---------------------+

4. What is your TiDB version? (Required)