Equidistant

Pick a year (the measure-from date's month and day in that year) or an exact date. Equidistant finds the mirror date: the day that sits exactly as many days before your pick as the measure-from date sits after it. The measure-from date defaults to today; choose any other date with the Measure from picker, and press Today to go back.

For both the picked date and the mirror date, it reads English Wikipedia live and shows:

• No. 1 song — Billboard Hot 100 chart week in effect (from 1958)
• No. 1 film — U.S. weekend box office (lists start around the late 1970s)
• Top TV show — highest-rated U.S. network program of that season (from 1950–51)
• Events — Wikipedia's "On this day" feed plus the year article, with nearby dates when that exact day is quiet
• Born that day

The month, day and year of all three dates (mirror, picked, today) are editable in place. Changing the mirror moves the picked date to the halfway point; changing today recalculates from a different reference day.

Single file (index.html), no build step. Serve it with GitHub Pages (Settings → Pages → Deploy from branch → main / root). The Share button copies a link to the exact combination on screen, or a text summary with both dates' songs, films, shows and events. A shareable link is ?date=YYYY-MM-DD or ?year=YYYY, plus &today=YYYY-MM-DD for a different measure-from date.