The date of summer solstice varies from year to year (as does winter solstice and both equinoxen) because the year isn't exactly 365 days long. It can also vary on any particular instance based on what side of the date line you're on, but expressed in GMT it happens at one particular second worldwide.
I told my kids, "Someday, you'll have kids of your own." One of them said, "So will you."
-- Rodney Dangerfield.