Or to look at it another way, 20 days after 21 June is how much light out you will have much like today.
For 7 years I worked at a Canadian Army Cadet Training center, two months at a pop in Whitehorse Yukon. When we had leave, Skagway was our usual destination. I would always leave roughly the 20th August. But on the 20th of August in Whitehorse, you still could go for a walk at 10:30 at night, still light left. When I flew down to North Vancouver, the 21st of August was always depressing as it was getting dark by 8:30.
During the summer in Whitehorse, an amazing number of people are out for a walk at eleven at night. And there is a book store that all the tourists hit when in Whitehorse on the main two block shopping area downtown that was open for quite a few years till midnight.
And to add a bit of boring trivia. The Canadian Forces took over the Alaska highway just at the end of WW2. My dad was an officer in the Royal Canadian Service Corp (which became Logistics), and he was the one who ran the supply, transport, and structures in an around Whitehorse. There was at one time, roughly behind where the RCMP detachment is now, the army hospital where I was born.
So I use to tell my military friends while I was in the Forces - I was Army Issue.
During the war, my mother was in communications with the Canadian Air force. Her job was to monitor and record Japanese submarine communications. No, she didn't speak Japanese. Apparently there were a number of German and Japanese Subs in and around the Alaska/BC coast.