Slippage is the amount of time the finish date of a task will occur or did
occur past its baseline, ie originally scheduled, completion date. Santa's
elves were scheduled to finish packing his sleigh by the evening of the 24th
but last minute repairs on the sleigh means it won't be ready to fly until
New Year's eve - the opening prezzies task will be delayed until New Years
day and so has 1 week of slippage.