Instead of using a edit validate rule use a validate rule in the flow action under validation. You can use function [a datetime] is in the [past/future] to validate and as per your requirement the date should not be in past
You can also write your custom validate message here.
If you want only with Edit Validate, write new edit validate rule like "NotPastDate" with the code below and use it
The getDate() method just return the day of the month (an int). If the day of the new date is equals or greater than the day of the current date the result always will be true and it won't matter if the day of new date belongs to a past month. Also the getDate() method is deprecated.