Albert Barnes Commentary 1 Samuel 18:26

"And when his servants told David these words, it pleased David well to be the king`s son-in-law. And the days were not expired;" — 1 Samuel 18:26 (ASV)

The days were not expired: David was so rapid in his attack on the Philistines that he was able to bring the required dowry and receive his wife, Michal, before the time had expired for him to receive Merab.