Statute of Limitations for Payment Collections

Hopefully some of the experts who deal with contracts can give me some tips
on this..

A job was completed in May 2003, and (for logical reasons too much to
explain) was not invoiced until April 2005.

I understand that normally there is a 2 year period after which no
collection/legal action can be taken. What I'm wondering is, in this case,
does this two year period start at the completion of the job or when the
invoice was sent?

Any comments or references to legal sources is greatly appreciated!

Fred