Since your problem is on multiple forms, you will have to repeat the solution for each form where that problem applies. So get it "right" for one form and then you will know how to do it for multiple forms.
Since the value in question is entered on a form but affects other forms, and I'm going to presume that this solution should be "persistent" - that is, applies even after you have closed and re-open the database in question, you need to actually store the closure date. I don't know your structure so cannot advise on WHERE to store it except that the only place to persistently store something is a table.
Therefore, on the form where you enter that limiting date, you must find a way to store the date in a table. I don't know that it matters if there is anything else in the table and I don't know what other requirements you might have. The SIMPLEST case is a one-record, one-field table - but that might be wasteful and you might already have another, more convenient place to store such a thing. We don't know, because your description of your problem was strictly limited to your desired effect, not your surrounding database.
In general, Arnel's proposed solution is right for the limited description you provided but he only superficially covered storing the date to achieve persistence across restarts.
In general, when you want us to write code for something, it helps if we know more about your database setup or environment. Having said that, the theory behind what Arnel proposed is this:
If you have a persistent copy of the closure date that you want applied, you can use the DLookup() function to retrieve the date. Having the date, you can make comparisons against it, such as on the forms you want to protect. When you are about to save the contents of a form, something called the "BeforeUpdate" event fires and if you can put this code in place to PREVENT the update (using the Cancel parameter that is part of that event), you can stop the form and issue a warning to the user. The user would not be able to save the data as long as the closure date on the form is wrong. In fact, this is the place that you would put ANY validation code for a given field on the form if you were testing for special constraints such as this.