Cloudwatch Query with dynamic timestamps

I am getting started on doing some simple Cloudwatch Queries in Redash. Unfortunately I don’t get expressions to work but at the very least I want to have a simple query be limited by a certain timespan.
I cannot use a variable parameter as I want to use the query for alerts.

For example:

StartTime: "2022-02-01 00:00:00"
EndTime: "`sdfsdfdfj`"
- Id: db001
      Namespace: AWS/RDS
      MetricName: FreeableMemory
      - Name: DBInstanceIdentifier
        Value: "db001"
    Period: 300
    Stat: Average

Works just fine however it does not seem to care about the value in end-time. What I really want is to just return the last value for that database.

Hope my request makes sense.