Why the months and days look uneven
Because calendar months aren't all the same length, your "months and days since your last birthday" figure isn't a clean 30-day chunk — it's the real distance on the actual calendar, leap years included. That's also why the total-days count is exact rather than an estimate: it's counting real days elapsed, not years × 365.
Nothing entered here is saved anywhere — the whole thing runs in your browser and resets the moment you leave the page.
What's the "days until next birthday" counting from?
It finds the next occurrence of your birth month and day from today — if that date already passed this year, it counts to next year's instead.