Are you doing the paper form or the online form?
If paper (recommended), the application date is the day of the appointment. So that would mean payslips from February 1st to July 31st (if paid at the end of the month). If the February payslip doesn't have "February 1st" on it, include the January payslip dated the end of the month to ensure you are covering the full period.
Once you have your window of payslips, ensure your bank statements cover the same date range (likely to be a bit longer on each side).
Employer letter dated within 28 days of the appointment as well.
If you completed the form online, everything changes based on the date you submitted the online application.