The royal dad visited the Royal London Hospital and was announced as the patron of the air ambulance's anniversary campaign on Wednesday, which happens to be his wife Kate Middleton's 37th birthday.
Prince William didn't need any reminders for today's special significance.
This year will be Kate's first birthday as a mother-of-three, after giving birth to Prince Louis on April 23, 2018.
More than 4,000 miles away, Princess Beatrice was enjoying decidedly warmer weather as she strolled hand-in-hand on an African beach with her new boyfriend.
The duke also mentioned his three children - George, 5, Charlotte, 3, and Louis, 8 months.
George, who is a big fan of helicopters, will be glad when he receives a plastic toy of the aircraft.
There was an extra present, a model helicopter, for George and his father quipped: "I can not go back without a helicopter, George will never forgive me". "George would never forgive me!"
The prince, who worked for the East Anglian Air Ambulance until July 2017, also praised the boy and girl, brother and sister, for their neat handwriting, saying: "Your writing is very good as well". His visit coincided with him being named patron of their service's 30th anniversary celebrations - the Duke of Cambridge previously served an air ambulance pilot for a number of years.
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