fix(core): Account age inaccuracy due to leap years

[spenserblack]

By converting the diff to a date, the date since 1970 is calculated. The
year of that date minus 1970 would then equal the number of years that
have passed, taking leap years into account.

Resolves #497

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As a side note, I wanted to write tests for this, but making all the computed values of the core plugin testable would require a lot of refactoring, it seems.

[spenserblack]

Closing to avoid spamming vercel

[lowlighter]

Thanks for fixing this!
Looks good to me 👍

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However I think the diff value may break the progress tracking of the "Member" achievement as it expects a float number of years:

metrics/source/plugins/achievements/list/users.mjs

Line 222 in cc709a5

const value = computed.registered.diff

(This way it makes it progress a little day-by-day instead of waiting a whole year)

But I'm not sure what's the best way to solve this 🤔
Maybe something along diff: years + months/11 + days/(lastMonthNumberOfDays) ?

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As a side note, I wanted to write tests for this, but making all the computed values of the core plugin testable would require a lot of refactoring, it seems.

Yeah that's ok. Testing metrics is quite hard because there are so many edges cases, and mocking apis and data is really tedious x)

[spenserblack]

However I think the diff value may break the progress tracking of the "Member" achievement as it expects a float number of years:

Oh, I see. I partially addressed the breakage in #501, but I failed to make it increase in daily with this method.