spatial statistics - How to get "p value" for time series data to detect significant change

I have yearly maximum NDVI data (1982-2008) and extracted all value to Excel. Also calculated slope value for each point during the 27 years. All of you know the slope value represents changes per unit (per year). Now I would like to look for significant changes in NDVI over the 27 years. So I have to get "p value" for each point. How I calculate it?

I tried it in Excel. Firstly I have to estimate t-test, but i don't know which value should be in place of 'mu' for this case! After estimating it I can find p value using TDIST function in MS excel.