Receiving backdated invoices

[likessleep]

Hi,
I just wondered how common this was? I've received one today, that is dated 30th April (which is theoretically when their 'work' finished - the agency recruit market research respondents). However, I think this is quite cheeky - it is their delay and is nearly 2 weeks late, leaving me 2 weeks to pay (they have 30 days payment terms).
I have just started up, so cashflow is a big thing for me at the moment. I knew the bill was coming, but I get paid at end of the month, so was glad they were late sending through the invoice, as it means I get paid first.
Am I over-reacting?
Should I say that I will pay within 30 days, but that as I have received the invoice today, that it will not be 30 days within their backdated invoice date?
Any thoughts appreciated - cheers!

Yes I would take 30 days.
I am self employed charge for my time. I go to clients premises so they know the date I am there, but they pay 30 days from receipt of my invoice.
It gives me incentive to get my invoices out quicker!

Thanks flibbertyjibbet. I just wanted to check I wasn't being unreasonable really. Thanks for your thoughts. If I was late / forgotten to send an invoice, I wouldn't backdate it, so that's why it's annoyed me.
Thanks

It's more likely they backdated it for accounting reasons than to expect to be paid.

Write on your invoices the date you receive them and pay them 30 days from that date.

thanks for posting legacy.
that's what i've decided to do - thanks, it's reassuring.