Duration Calculator for Dates in VB.NET

Hello Guys,
I was working on a project of a hotel management system. I had to calculate the day between two given dates by DateTimePicker in vb.NET.
I named the DateTimePicker as "DTPcheckin" for the day when customer checks into the hotel and
"DTPcheckout" for the date of checkout.
To calculate the date and to facilitate myself for the later calculation and SQL comparisons I converted the date to the NUMBER OF DAYS FROM THE BEGINNING OF THE CALENDER . 
LIKE "20 MARCH 2013" == 735328. that is 735328 days passed from date 01/01/0001.
U can use this anywhere you want. here is the code------


    Dim checkin_day As Integer
    Dim checkout_day As Integer
    Dim checkin_month As Integer
    Dim checkout_month As Integer
    Dim checkin_y As Integer
    Dim checkout_y As Integer
    Dim staying As Integer
    Dim mdcin As Integer
    Dim mdcout As Integer
    Dim daycin As Integer
    Dim daycout As Integer

 Private Sub calday()
        checkin_day = CInt(DTPcheckin.Value.Day)
        checkout_day = CInt(DTPcheckout.Value.Day)
        checkin_month = CInt(DTPcheckin.Value.Month)
        checkout_month = CInt(DTPcheckout.Value.Month)
        checkin_y = CInt(DTPcheckin.Value.Year)
        checkout_y = CInt(DTPcheckout.Value.Year)

        mdcin = mdcount(checkin_month)
        mdcout = mdcount(checkout_month)
        daycin = checkin_y * 365 + mdcin + checkin_day
        Dim temp2 As Integer = checkin_y - 1
        temp2 = temp2 / 4
        daycin = daycin + temp2

        If (checkin_y Mod 400 = 0 Or (checkin_y Mod 100 <> 0 And checkin_y Mod 4 = 0)) And checkin_month > 2 Then
            daycin = daycin + 1
        End If

        daycout = checkout_y * 365 + mdcout + checkout_day
        Dim temp As Integer = checkout_y - 1
        temp = temp / 4
        daycout = daycout + temp
        If (checkout_y Mod 400 = 0 Or (checkout_y Mod 100 <> 0 And checkout_y Mod 4 = 0)) And checkout_month > 2 Then
            daycout = daycout + 1
        End If
//duration is calculate by subtracting the daycin from daycout
duration = daycout - daycin
    End Sub

//the mdcount function is
 //this  mdcount function calculate the number of days from 1st jan to the given month
 Public Function mdcount(ByVal n As Integer) As Integer
        Dim md As Integer

        If n = 1 Then
            md = 0
        ElseIf n = 2 Then
            md = 31
        ElseIf n = 3 Then
            md = 59
        ElseIf n = 4 Then
            md = 90
        ElseIf n = 5 Then
            md = 120
        ElseIf n = 6 Then
            md = 151
        ElseIf n = 7 Then
            md = 181
        ElseIf n = 8 Then
            md = 212
        ElseIf n = 9 Then
            md = 243
        ElseIf n = 10 Then
            md = 273
        ElseIf n = 11 Then
            md = 304
        ElseIf n = 12 Then
            md = 334
        End If
        Return md
 End Function