Thanks,
Not sure it works though - when creating the script and running it everything seems normal, but when I look in the logfile afterwards I see that
Modify xxxxxxx (yyyyyyy): clear Account Expires Set expiry date 2018-03-22 21:20:08
Checking the user the account expires field is still empty .... so from where does the clear account expires come?
But changing the modify user field to LastLogonTimestamp +1 month did the trick.