How to Creating Your Own Functions
As you might already know, Access has lots of built-in functions that you can use in creating expressions. When you use the Expression Builder in Access to create an expression, you can view a list of all the built-in functions and also choose any function that you want to incorporate into the expression you're writing.
First, you need to get to a place where one might actually write an expression. For example, if you're designing a table and decide to set a Default Value for a field, as soon as you click the Default Value field property, a Build button appears. Clicking that Build button opens the Expression Builder. If you set the Default Value to =Date(), =Date() is an expression that uses the built-in Date() function to return the computer's date.
You can also use expressions to create calculated fields. You can also use expressions to create calculated controls on forms, where the control's Control Source property contains an expression that does some math or returns some value based on other data in the same form.