It will be way easier to time your contractions when you are in labour, don't bother timing them while they aren't regular or you'll do your head in! (I did this in early labour with DS1 and it was sooo annoying and disappointing).
Yep - while ever you're having trouble identifying beginnings and endings, or while they're really irregular or far apart it's not worth timing them. It just makes you focus on it and in early labour you don't want to be doing that. When you need to be timing them they will get your attention quick smart