result of unix_timestamp() is not consistent with MySQL

[zz-jason]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

In TiDB:

TiDB(localhost:4000) > select unix_timestamp('2018-01-01 18');
+---------------------------------+
| unix_timestamp('2018-01-01 18') |
+---------------------------------+
|                               0 |
+---------------------------------+
1 row in set (0.01 sec)

2. What did you expect to see?

In MySQL:

MySQL(localhost:3306) > select unix_timestamp('2018-01-01 18');
+---------------------------------+
| unix_timestamp('2018-01-01 18') |
+---------------------------------+
|                      1514800800 |
+---------------------------------+
1 row in set (0.00 sec)

3. What did you see instead?

as section 1 describled

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

TiDB(localhost:4000) > select tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v2.1.0-rc.3-56-ge544882d0
Git Commit Hash: e544882d0b8c4cf4ec1b93722837d2821950bb9a
Git Branch: master
UTC Build Time: 2018-10-17 09:48:11
GoVersion: go version go1.11 darwin/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false
1 row in set (0.01 sec)