Why Does JavaScript Parse the Same Date Differently Across Time Zones?

I’ve run into a confusing date bug: a date string that looks like a calendar date can show up as the previous day on a computer in another time zone. The value seems harmless, but the displayed result changes depending on where the code runs.

What does JavaScript assume when it parses a date-only string, and how should an app represent a date that has no time or time zone attached? I’m especially interested in avoiding this problem for birthdays and booking dates.