Accessing Subs from another Activity

Hi,
This may sounds simple for the experienced ones...

How do I access a function/subroutine from another Activity?

For example, I have a sub called GetJulianDate in "Page1" activity. Let's say I need to access the same GetJulianDate statement in current "Page5". How do I call the GetJulianDate that is in "Page1"?

I tried calling GetJulianDate in Page5 but it doesn't work. It doesn't recognise there was a GetJulianDate in Page1. If i inserted identical GetJulianDate sub in Page5, then it works. But this creates duplicated subs in all activities.

Thanks!