I've been here for 19 years, 11 months, 4 weeks, 1 day . . . is this a math error on my end, or on Subeta's? Shouldn't it be 20 years by now? Four weeks in a month, 12 months in a year?

It goes precisely by the date you joined and accounts for leap days, so it ticks over at more like day 2/3

4 weeks doesn't actually equal a month, unless that month is february.

A month is accounted as in 4 weeks and 2 days, isn't it?

Since most months, except February, have at least that.

Yep, yesterday!