No, the date is mentioned in description. If the description has August date then it will be counted in capping of August month.
View attachment 65610
The date mentioned is 16 August so it will be considered in this month's capping.
The date of posting of reward points doesn't matter. This was posted on 19 August. Even if this was posted on September with date mentioned as 16 August then it would not have been included in September's quota.
If you had already reached previous month's quota and rewards are posted of back date then 0 RPs would have been credited with description saying maximum cap reached.