Days since your birthday
Counts the days since your most recent birthday, and the days remaining until the next one. The two always add up to the length of that birthday year — 365 or 366.
How to use the days since your birthday
The two numbers together are a small sanity check on the calendar: days since plus days until equals the length of the year that birthday sits in. If your birthday falls after 29 February in a leap year, that total is 366; otherwise 365. It is the clearest illustration of why "a year" is not a fixed number of days.
Counting since rather than until is what you want for anything measured from an anniversary — a warranty that started on the day, a subscription that renews annually, a probation period. In all of those the count that matters is elapsed days, and it is worth knowing that the count is inclusive of neither end: the day of the birthday itself reads as zero, and tomorrow reads as one.
For a leap-day birthday the same 1 March convention applies as everywhere else here, which means the count since resets on 1 March in a common year and on 29 February in a leap one.
Questions
Yes — to 365, or 366 if a 29 February falls between them. That is the clearest demonstration that a year is not a fixed count.