In an Access form, I need to use the date output from a date picker field to be used to search for a file in a form. However, the default data format seems to only allow slashes
The Format Function will allow you to format the date as you wish.
An expression to include the formatted date in a filename pattern could look like this ...
"myfilename_" & Format(Me.myDate, "yy-mm-dd") & "*"