Nick's tutorial. 
My examples there actually do have the creep which is mentioned earlier. Putting aside coding issues, this creep is based on the fact that you reset the timer when the event is noticed, not a fixed interval from when it should have been noticed.
In the case of flashing an LED, this isn't an issue.
Let me give you an example in real-life terms. Say you get a credit card statement in the mail, and you have 30 days to pay. The 30 days is from when the statement was sent, not from when you receive it (which might be a few days later). The creep is in the delivery time (or maybe in the time it takes you to notice the mail).
To have the statements come every 30 days, the sender simply adds 30 days to when they sent the last one. To have the creep, they would add 30 days to when you received the mail.