sql查询时间戳是15天以前

i have this SQL query:

where datetime < DATE_ADD(DATE(now()), INTERVAL 15 DAY) order by datetime ASC 

is this the right query to say where datetime is 15 days or more ago?

You should use DATE_SUB instead:

where datetime < DATE_SUB(DATE(now()), INTERVAL 15 DAY) order by datetime ASC 

• DATE_ADD "adds" an interval to the date you start from.
• DATE_SUB "subtracts" an interval on the date you start from.

See here for documentation.