Hi Luke
Welcome to the forum, you might want to have a look at the following FAQ which explains where everything is.
Welcome to Benefits and Work .
It will depend on which decision we are talking about.
For PIP:
If it is a new claim then and you returned the claims form within the time limit then the claim will be backdated to the date on the form.
If it is a new claim and you sent the form in late then it will be backdated to the date DWP received the form.
NS ESA: end of the 13 week assessment phase.
Gary
