Ms. Grimes sent a letter to Mr. Howard, offering to sell him her collection of Canadian oil paintings.Her letter stated the following:

MEMO FROM: R. Grimes

TO: T. Howard

DATE: January 1, 2016

I hereby agree to sell to T. Howard my entire collection of Canadian oil paintings (6 paintings in total) for the sum of $1 million. This offer is open until Monday, January 4, 2016, at 9:00 a.m.

On Sunday, January 3, 2016, Ms. Grimes decided to sell the paintings to her well-to-do neighbour instead. Mr. Howard heard about the alleged sale later that same day and rushed over to Ms. Grimes house, stating that he wished to accept the offer. Ms. Grimes smiled and said "Sorry, you're too late. I've sold to someone else."

Is Ms. Grimes obligated to keep the offer open until the specified time? What could Mr. Howard have done to better protect his position?