Replace:
Dim sqlstring As String = SELECT firstname, lastname, startdate, starttime, endtime, hours FROM hoursworked WHERE startdate = Calendar332.SelectedDate AND startdate = Calendar333.SelectedDate ORDER By startdate
with:
Dim sqlstring As String = "SELECT firstname, lastname, startdate, starttime, endtime, hours" & vbcr & _
"FROM hoursworked" & vbcr & _
"WHERE startdate BETWEEN '" & Calendar332.SelectedDate & "' AND '" & Calendar333.SelectedDate "'" & vbcr & _
"ORDER By startdate"
For further information, please see:
String Data Type (Visual Basic)[
^]
[EDIT]
SELECT firstname + ' ' + lastname AS [Employee], SUM(DATEDIFF(day, endtime, starttime))
FROM hoursworked
WHERE startdate BETWEEN @firstdate AND @seconddate
GROUP BY firstname + ' ' + lastname
ORDER BY firstname + ' ' + lastname
[/EDIT]