SPM170400 | Average Weekly Earnings (AWE) -SSP: relevant period not a whole number of calendar months
From HM Revenue & Customs · Statutory Payments Manual
If an employee is normally paid once each calendar month:
Establish the relevant period.
To give a whole number of calendar months in the relevant period count the number of odd days and then round these up or down as follows:
any month except February: round 15 days or less down, 16 days or more round up
February: round 14 days or less down, 15 days or more round up
add together the gross earnings in the relevant period.
Then use the number of rounded months as at employee paid monthly step 4 SPM170300.