Can enter custom value in Outlook reminder field
As you noted, Outlook's reminder duration pull-down field has several pre-defined values, such as "5 minutes", "10 minutes" and "2 weeks". However, you can enter a custom value in this field. I regularly enter values such as "7 weeks", and they work. So, to get a reminder two months before a due date, enter "8 weeks".
You can enter other units in this field. For example, you can enter "1 month" or "3 years" and press the "tab" key, but I do not know if they work as expected. I have not tested these units.
Interestingly, if you enter a "months" or "years" value and press the "Enter" key (instead of pressing the "Tab" key), Outlook complains with a dialog box that says, "The units in the duration are not valid." After you press the "OK" button, Outlook highlights the contents of the duration field without changing it.
If you enter another unit (such as "quarter" or "decade") and press the "Tab" key or press the "Enter" key, Outlook automatically changes the unit to "minutes" without a complaint dialog box.
I suspect that Outlook treat "months" and "years" correctly (if you press the "Tab" key after entering a value), but I have not tested my theory.