Q:Given the following passage  "The Royal Navy is constructing two new larger STOVL aircraft carriers, the Queen Elizabeth class, to replace the three now retired Invincible-class carriers. The ships are HMS Queen Elizabeth and HMS Prince of Wales. They will be able to operate up to 40 aircraft on peace time operations with a tailored group of up to 50, and will have a displacement of 70,600 tonnes. HMS Queen Elizabeth is projected to commission in 2017 followed by Prince of Wales in about 2020. The ships are due to become operational starting in 2020. Their primary aircraft complement will be made up of F-35B Lightning IIs, and their ship's company will number around 680 with the total complement rising to about 1600 when the air group is embarked. The two ships will be the largest warships ever built for the Royal Navy.",  answer the following question. Note that the answer is present within the text.  Question: what time period is mentioned last?
A:
2020