When you add a new post, use NOW() to insert the current date/time to your field like
Or use TIMESTAMP column for which the default value is CURRENT_TIMESTAMP. In your table declaration such a field could be declared like:
INSERT INTO mytable (field1,field2,filed3,post_time1) VALUES ('a','bc',2,NOW());
Please notice you could have only 1 such TIMESTAMP column in a field. Please also notice that MySQL will automatically update value (with the current timestamp) in this column each time you update any field in the table row.
post_time1 TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
If this not suitable, please use NOW() on insertion as described above.
Edit: personally I would use NOW(). This is an insertion date, not last edit date, right? So most probably you do not need the date to change each time the record is edited. I would simply use NOW() on insertion.