What is the value of “X” in a regression equation when dealing with a time series?

I am using excel to add a polynomial trend line to a chart. The chart and the formula of the trend line are shown below. I want to add lines indicating different confidence intervals so I need to find out the y values representing the trend line drawn on the chart.

If I had 2 columns of "normal" data - e.g. height and weight - I understand that I could just plug in the x value into the equation and get the result of the trend line. However, in this case my x axis is a series of dates. What is the value of x that you would plug into the equation to generate that line? I can't plug in a date (or can I?) and if I play with the formatting, excel turns the dates into very large numbers (my guess is number of days since a certain start date). What I am missing? Is there an easier way to do this?

Thank you • Confidence intervals for such a fit would be meaningless: those data certainly don't follow such a trend and it's not even a good qualitative description. – whuber Nov 22 '14 at 1:25
• @whuber Ok I just wanted to see if I could calculate them. Can you suggest a better method? I don't have much experience outside of a normal linear regression and I normally use R not excel. This just don't look linear at all. – Rarw Nov 22 '14 at 15:56