I’m sorry for the “re-save”. You can write a script to help you doing this.
datetime field, if you set
'std' => ' ', then when saving the post without specifying the datetime, it will save exactly as
' ' in the database. Actually, the plugin treats datetime field like a text field, unless you set
'timestamp' => true, which converts datetime into integer. And then in that case, the value is kind of unexpected, cause
' ' is converted to incorrect value.