sql update random between two dates

I've tried to update my database and changing dates. I've done some research but I did not found any issue. So I used two timestamp.

I've tried to do that method:

UPDATE `ps_blog_post` 
SET `time_add` = ROUND((RAND() * (1387888821-1357562421)+1357562421))

Now everywhere the new date is:

0000:00:00

Anykind of help will be much appreciated