I have a philosophy on birthdays. The significant ones aren’t the numbers we usually choose- 18, 21, 40, whatever- it’s the ones where you need an extra bit. 2, 4, 8, and so on. By that standard, my next birthday landmark isn’t until 2044, and I’m a patient sort.

Christian inherited some legacy C# code which deals in birthdays. Specifically, it needs to be able to determine when your last birthday was. Now, you have to be a bit smarter than simply “lop off the year and insert this year,” since that could be a future birthday, but not that much smarter.

The basic algorithm most of us would choose, though, might start there. If their birthday is, say, 12/31/1969, then we could ask, is 12/31/2020 in the future? It is. Then their last birthday was on 12/31/2019. Whereas, for someone born on 1/1/1970, we know that 1/1/2020 is in the past, so their last birthday was 1/1/2020.

Christian’s predecessor didn’t want to do that. Instead, they found this… “elegant” approach:

static DateTime GetLastBirthday(DateTime dayOfBirth)
    var now = DateTime.Now;

    var former = dayOfBirth;
    var current = former.AddYears(1);

    while (current < DateTime.Now)
        former = current;
        current = current.AddYears(1);

    return former;

Start with their birthdate. Then add one to the year, and store that as current. While current is in the past, remember it as former, and then add one to current. When current is finally a date in the future, former must be a date in the past, and store their last birthday.

The kicker here, though, is that this isn’t used to calculate birthdays. It’s used to calculate the “Start of the Case Year”. Which operates like birthdays, or any anniversary for that matter.

var currentCaseYearStart = GetLastBirthday(caseStart);

Sure, that’s weird naming, but Christian has this to add:

Anyways, [for case year starts] it has a (sort of) off-by-one error.

Christian doesn’t expand on that, and I’m not entirely certain what the off-by-one-like behavior would be in that case, and I assume it has something to do with their business rules around case start dates.

Christian has simplified the date calculation, but has yet to rename it: it turns out this method is called in several places, but never to calculate a birthday.

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