That's a good one! I was just thinking about this yesterday as I watched the sunset. The variable rate of daily change in sunrise/sunset times according to the date with respect to the equinox and solstice is where I run out of math too.
Will wait for Gengis, Mad flt scientist, or J.T. to read this and formulate a response. They can usually be relied on for this kind of thing. Or someone may beat them to the draw!
Best regards,
Westhawk