Private Sub CommandButton1_Click()
Dim dt1 As String, dt2 As String, d As Double, mHours, mMinutes, mSeconds
dt1 = TextBox1.Value
dt2 = TextBox2.Value
d = DateDiff("s", CDate(dt1), CDate(dt2))
mHours = Format(d \ 3600, "00")
mMinutes = Format((d - (mHours * 3600)) \ 60, "00")
mSeconds = Format(d - ((mHours * 3600) + (mMinutes * 60)), "00")
TextBox3.Value = mHours & ":" & mMinutes & ":" & mSeconds
End Sub