Possible bug in Hipparcos calculations regarding decimal years and leap years

[sefffal]

Describe the bug I'm working on hipparcos modelling in my own code and ran into this problem. I thought I'd check how orbitize! does it, but I think we might both have the same error.

The hipparcos IAD residual data gives times in units of "1991.25 + year". In the hipparcos file, currently the code does:

times = iad[1] + 1991.25
# later...
epochs = Time(times, format="decimalyear")

The problem is I don't think that the offsets computed as 1991.25 + fractional_year should be considered a decimal year time. This is an issue only for scans between 1992 and 1993, because 1992 is a leap year with an extra day (366 days instead of 365).

cc @semaphoreP