newbee question

I want to create some extension similar to date extension.  I looked
at the code for date extension but i could not figure our how it
works.  It uses 'function-available("date:date-time")'and i could not
find this particulat function anywhere in the imported files.

Can someone please point me to a document or link where i can find
information about developing extensions?

I will appreciate your help.

Thanks!!!