The reason you're unable to save the policy before 9:00am EST is due to policies being configured using a daily metrics interval.
Platform metrics begin a 'rollup' process at 9am EST every day and that rollup can take up to an hour (or more depending on your infrastructure). If you were to set a policy to trigger at 6am EST for example, you would get an alert that is 2 days behind as the platform did not have enough time to rollup the previous day's metrics.