Delay in raising invoice?

hello we provide training services to corporate. what is the time limit within which we have to raise the invoice? We usually try to raise the invoice to the client within 30 days of date of conducting the training program.

This time we delayed raising the invoice by 11 days. Training program conducted on 17th jan and we will be raising the invoice on 28th Feb. what this be a problem? Shall we raise a pre dated invoice or 28 Feb is just fine?

Thanks!