InfoPath Dev

 I am sure that this is probably SO easy but I'm not a programmer and always have problems with syntax in a formula.

(all is based on SharePoint Online and InfoPath 2013) 

I have a field in a form with a display name of  "Detention Date Requested" (actual name: "DetentionDate").

I want the user to put in the Detention Date Requested, click on a button called "Check if Date is Available" where the button actually causes the action of going out to the list "Detention List 2015-2015", which I already have the data connection, counting the dates that match the date the user just entered and placing that number into a box where it now looks to see the value and if =25 gives a validation error that there is no more space available on this day.

So in essence (bad syntax here I know), when the button is clicked the formula is something like this:

count (DetentionDate)where(Detention Date Requested)=(DetentionDate) and return the number

If you set the query field in your Detention List data connection to the selected date, then execute the query, you should get back just the rows that match. No need to filter in your count formula. :)

 That returns each item but not a count. I only need to know how many records/list items currently exist with that date.

Right, that returns the items that match. Then you use the count function to count them....

 Thank you Hillary. You helped me resolve the issue by just adding the COUNT() in the formula, but also telling me to add the query. You're the best!