How to Clear the Scheduled Date for all WordPress Draft Posts

Use this SQL Query

Set the value in  the post_date_gmt field of your wp_posts table to the default value.

Run the following line of code in your phpMyAdmin Query box.
If you have changed the prefix of your WordPress tables in
your database, be sure to change wp_ prefix to match yours.
UPDATE wp_posts SET post_date_gmt = '0000-00-00 00:00:00' WHERE post_type ='post' AND post_status = 'draft'

How to Access phpMyAdmin on Your Web Host

If you need help accessing phpMyAdmin on your host here is how you can find help: Help With Using phpMyAdmin