Here's my suggestion:

1. Download Greg Pelletier's excel file (filename "twilight.xls")
Greg's page, get the 6th link down:
http://www.ecy.wa.gov/programs/eap/models.html

2. Open the file, and Save-As an Excel Add-In file (new filename "twilight.xla")
Basic add-in info I referenced to get it working:
http://www.fontstuff.com/vba/vbatut03.htm#addin

3. Install in add-in.
For me / Excel 2013:
File > Options > Add-Ins > Manage: Excel Add-ins (dropdown) > Go... > Check the "Twilight" box*

4. You're done. The sunrise, sunset, and the rest of greg's functions should now work just like any other function, in all of your workbooks as long as the add-in is installed. Refer to the twilight.xls file for instructions on using the functions, but basically, just type "=sunrise(lat, lon, year, month, day, timezone, dlstime)" to return the time of sunrise.

*if the new add-in doesn't populate automatically: I think you need to save the .xla to your app data folder (User/AppData/Roaming/Microsoft/AddIns). Google will have more on this. It wasn't an issue for me...

Thoughts: Greg has done some really great work! No changes are needed whatsoever to get this working; just save as an add-in, install and you're done. Now "sunrise" and "sunset" are functions available in all your workbooks. No need to add VBA code or new functions to each work book. I always forget the order of the inputs, but this is in the twilight.xls and is very clear.

I use an If/And structure with greg's VBA to check if time is during the day or night. Works like a charm!
If(And(Sunrise<CurrentTime, CurrentTime>Sunset),Day,Night)

Hope it helps!