Convert Text To Date

Hi, I get the user input from a text box in a form, then i use it in the query.

in my query i used the DateDiff function in the expression. should I convert the data type from string (the user input from the text box) to date type first before using the DateDiff function?

in the SQL view, I wrote the following, but it says incorrect:

SELECT availability.machineName,
(DateDiff("d",CDate(Forms!frmMain!txtStartDate),CDate(Forms!frm Main!txtEndDate))+1)*24 AS totalMonthlyHours, availability.type
FROM availability
WHERE (availability.date)>=CDate(Forms!frmMain!txtStartDate) And (availability.date)<=CDate(forms!frmMain!txtEndDate)
GROUP BY availability.machineName, availability.type;

Thanks in advance.