I have collected proportion-over-time data very similar to this published graph: and I'd like to compare my data with the fitted curve.

The paper gives the fitted quadratic as Y = 598809 – 31.075X + 0.0004X

^{2}.

I'm trying to figure out what format I need to convert my dates into to fit a quadratic on the same scale. (Wouldn't be my first choice of model, but I'm trying to compare with the old paper).

That quadratic has a root at 42280.960215058, and I cannot figure out how to make 15 July 2005 be even roughly that number. By X=42281.3 Y i's already over 1, so the entire x-axis with about 50 days has to fit between 42280.9 and 42281.3 - dafuq?

The paper's analysis was done in SPSS, which apparently uses Lilian format (number of seconds since October 14, 1582). So it's not that.

In Excel (well, LibreOffice), 42280 is 03/10/15, whereas 15/07/05 is 38548 - so it's not that (number of days from 1 Jan 1900) either.

I'm 99.9% sure no fraud has been committed here, but I cannot figure out how whichever program was used to fit the curve was treating the dates. Anybody got an idea?

Thanks in advance.