I think, as long as the date that the bank statement covers up to is before the online application, you should be okay. So, if the statement ends on 2nd January, but it wasn't stamped until the 13th, I think that's all right.
Basically, the 6-month (or 12-month) period that you are using to meet the financial requirement must end BEFORE the online application date.
You can't submit the online application and then try to rely on a payslip or bank statement issued after that date towards your 6 or 12 months.