I haven't turned it into a standalone program yet, but I wanted a way to calculate praying the Orthodox Book of Hours according to a sundial instead of a normal clock, and I used excel to do it.

I found the formula for determining sunrise and sunset using latitude and longitude from NOAA's website. This allowed me to generate a sunrise/sunset table for my location by day.

I use VBA to update a clock every 10 seconds (I didn't need it to be more accurate than that) that feeds to a cell, and I use that to know when it is time for First Hour (sunrise), Third Hour (9 am, or 25% of the day), Sixth Hour (50% of the day), and Ninth Hour (75% of the day).

I have it all working and it runs in Excel on my work computer. My goal is to translate the logic into an app on my phone so that I can get notifications when it's time to pray. It's been a fun project, which included a side project of making a 9-LED display in Excel for the clock. I have a clock for normal world time, and a clock for sundial time.

It's been a fun project!