[SigmaPlot] - X units when regressed on time?

My data structure follows:

X data are dates, say March 29, 2016 to July 12, 2016. There are 105 days in this date range.

Y data are percentages (0-100% of nitrogen released from a slow release fertilizer).

I run a regression (quadratic polynomial), and it gives me an equation of course. The equation is of the form y = y0+a*x+b*x^2, where y0=-1.855e+7, a=8.721e+2, and b=-1.025e-2. The curve fit data show that the unit for X is 9 hours, 50 minutes, and 37 seconds (or 0.41 days). I want to report this equation in a manuscript, but I can't say that the units for X are so many fractions of a day. I need the units for X be "days" (as in, 21 days after applying fertilizer, how much nitrogen has been released?). I've tried reducing the 'step size' (Regression wizard, Option) to 105, but that doesn't work of course. I hope I've adequately described the problem.

A sample of the data I've described follows (date followed by percentage N released on that date):
3/29/2016 0.0000000000000000
4/5/2016 26.4150000000000000
4/12/2016 41.6650000000000000
4/19/2016 51.1700000000000000
4/25/2016 54.5825000000000000
5/10/2016 64.8660000000000000
5/31/2016 84.4975000000000000
6/21/2016 88.6650000000000100
7/12/2016 94.8325000000000000

Thank you for your assistance.