Any time I have something with an expiration date or an absolute deadline, I use Remember the Milk to store that date. This goes for both short term things like a bill I need to pay, as well as long term things like a car warranty expiration or a interest-free financing plan ending. In these cases I need to store the specific date since it's inconvenient to look up elsewhere, but I also need a bit of a heads up. It wouldn't do any good to be alerted the day that my car warranty expires, I need to have some lead time to check my mileage, get an appointment at the shop and to shop around for extended warranties. However, if I get alerted well in advance, the chances of me being ready to deal with it on that specific day are pretty slim.

I don't want to clutter my list by creating both a 'warning' task and a 'due' task, so what I do instead is create the task with an earlier due date to get alerted, and put the actual due date in the task title. That way, when the time comes around, I'll get alerted way ahead of time and I'll have the chance to 'snooze' the task a few times, but I'll always see at-a-glance the actual date that I cannot snooze past.