Ticket field: Dropdown and multi checkbox timestamp report

Hi Team,

How can I generate a timestamp report for a dropdown and multi checkbox field in the ticket (under a form).

I've tried using the following but it I keep on getting an error when adding the attributes.

Thanks

IF ([Changes - Field name]="{your_checkbox_field_name}")
AND
(([Changes - Previous value]="0") OR
([Changes - Previous value]=NULL))
AND
([Changes - New value]="1")
THEN
[Update - Timestamp]
ENDIF

and 

IF ([Changes - Field name]="{your_checkbox_field_name}")
AND
(([Changes - Previous value]="0") OR
([Changes - Previous value]=NULL))
AND
([Changes - New value]="1")
THEN
[Update - Timestamp]
ENDIF