[crispyambulance]

After you gain experience from previous slipped deadlines, you can see patterns that lead to slippage and, more importantly, get wise to the smell of unrealistic expectations. That's how you know that deadlines are going to slip.

[ljw1001]

you're assuming that deadlines are set based on the amount of time the work requires. Not often true. Generally deadlines are set before anyone knows how much work is required.

[mannykannot]

That's one of the patterns.