Hi
Just had a quick look at your test file ...
Working with Text in UserForms can be tricky for dates ...
Your Dateserial function does requires Numbers to work properly ... and very probably your inputs in your different ComboBoxes are simply Text ... which would need to be converted to Numbers with CInt() or CLng() ....
You could test following modification
Code
dt1 = DateSerial(CLng(ComboBox2.Value), CLng(ComboBox3.Value), CLng(ComboBox4.Value))
dt2 = DateSerial(CLng(ComboBox5.Value), CLng(ComboBox6.Value), CLng(ComboBox7.Value))
Hope this will help