For a possibly better 80x80 one (too.. many ... details, even for 80x80), which shows the days of the calendar more clearly, I can refer to the original 128x128 icon source file. Hang on, I'll attach it (128x128, larger than 80x80 so NOT an icon for us). It was found in a JREMIND.JAR of a possibly different JRemind. The size suggests it's the icon; there was more artwork, including a 60x60 image.
Our JREMIND.JAR doesn't contain an icon file, AFAICT, and I couldn't find its possibly updated and/original source files.
I've played with the days while resizing to 40x40, and may give it a third try later today. Now even 80x80 is too small. Please note that it's an alternative to grab any rather clear 3D'ish calendar icon and an own J.