Start of the hourly time window for Savings Plans.

The time window for calculating the money spent in Savings Plans is always based on the start of the hour (e.g., 13:00, 14:00, 15:00, etc.) and not on any arbitrary time within the hour (e.g., 13:05, 14:05, 15:05, etc.). This is because AWS calculates hourly usage and charges based on a fixed time interval of one hour, starting from the beginning of the hour.

https://docs.aws.amazon.com/savingsplans/latest/userguide/sp-applying.html

AWS documentation states that Savings Plans are charged hourly and usage is calculated hourly, which means that the usage and charges are based on the number of hours used within a specific time window. The Savings Plan hourly usage and charges are calculated by aggregating the number of seconds of usage within each hour, and the usage is charged at the hourly rate that applies for that specific hour.

Therefore, it is recommended to monitor your Savings Plan usage and charges in hourly granularity to get a clear understanding of your costs and optimize your usage accordingly.