Why summers?
Ask someone how many days they have left and the number is so large it means nothing. Ask how many summers, and the answer is suddenly something you could count on your fingers and toes a few times over. If you're 35 and plan for 80, it's about 45. That's 45 more long evenings, 45 more chances for the trip you keep postponing, 45 more times the people you love will be the ages they are this year.
Concrete units make time feel close. Behavioural researchers have found that the more concretely we picture a period of time, the more we treat it as real and near rather than abstract and far away. A summer is about as concrete as time gets.
How it's calculated
We treat each summer as June 21 to August 31. A summer you've lived all the way through counts as "had". This year's summer counts as "left" until it's over, because it isn't over. Future summers are counted up to the lifespan you choose (80 by default). It's a northern-hemisphere calendar, but the count is the same wherever you are: one summer a year.
The lifespan is a planning number, not a prediction. Recent US life expectancy estimates are in the high 70s, many countries are in the low 80s, and averages say nothing certain about one person. Pick the number you'd like to plan around.
A smaller number is a useful number
Gather ye rosebuds while ye may, old time is still a-flying.Robert Herrick, 1648
Herrick's poem is usually read as wistful, but it's really practical advice: the season is now, so go outside. Knowing you have a countable number of summers doesn't have to be sad. Studies on savouring suggest the opposite: people who are reminded that a good period is limited tend to enjoy it more and do more with it.
Some people use the number to plan backwards. If there are maybe 40 summers left, which ones are for learning to sail, which for the long road trip, which for the kids before they're teenagers? Some notice the number of summers left with their parents is far smaller than their own. And some just use it to say yes to the barbecue.
For a finer-grained version, see your weekends left, or the full picture in the life in weeks chart.
Keep the next one in view
Days Unspent is a live desktop wallpaper that keeps this kind of number in the corner of your eye: the days you have left to spend, your weeks as a spiral, and the one thing you're working on now. Your summers here are free. The wallpaper is $29, once.