Best way to calculate statistical intervals from data?

It's been a while since college statistics and I have some data for work I need to present. What is the best way to do an interval ysis on a group of data that includes # of days between two points in time? I'm wanting to find the average # of days as well as the intervals so I can see where future time frames may lie on the distribution. I have access to SPSS, JMP, Excel, and of course the good ol' pencil and paper.