Smys123,
I would suggest you create a simple table, one field (Date/Time data
type), one record, which is where the date is recorded for when the
checkbox is checked to Yes. So, on the After Update event of the
checkbox you could have a macro or procedure that would run an Update
Query to set the value of the date in this table to Now().
Then, on a suitable event in your database (I don't know enough to be
able to suggest specifically... maybe this is every time the database
gets started, or when a certain form is opened), you will need a further
macro or code, that will check the data recorded in the table, see if it
is more than 30 days ago, and if it is, run another Update Query to set
the Yes/No field back to No.
Having said that, this is an unusual thing to do, and in fact it is
somewhat suspicious... you are in effect, if you think about it,
replicating the same information in two different ways, i.e. whether the
Yes/No field is Yes or No, and whether 30 days has elapsed. I would be
interested to know why you need to do this.