Start Date
(closed account) says:
While due date is great, there are some times I want to see something on my daily list before it is actually due. One example is for real estate taxes, they are due the 15th of Oct, but obviously I want that paid before the due date. While I could set the due date earlier, that isn't an accurate deadline either. In this example I could set the start date of Oct 1st and a due date of Oct 15th.
I have long created tasks titled "taxes due 10/15" and given them a due date of 10/1, but... if you want something better than that, there are several threads on this issue, and while you wait for RTM to add this feature, may I suggest you look into the use of "sleeper" tags and smartlists to bring much of the functionality you wish to see to RTM today, as it is. I realize it's a bit of a kludge, but it does work pretty well.
C.f., http://www.rememberthemilk.com/forums/tips/7427/
C.f., http://www.rememberthemilk.com/forums/tips/7427/