Hi @ BenjaminDeMers,
Do you want to post a tweet in Twitter about all new created feeds in RSS from a special time or from a special count?
I would offer you a workaround to about your situation to post a tweet in Twitter about all new created feeds in RSS from a special time, you could refer to screenshot below to create the flow:


The expression in the condition would filter all feeds which is published later than UTC time “2018-03-23 09:00:00Z”, the expression in the condition as below:
@greater(items('Apply_to_each')?['publishDate'], '2018-03-23 09:00:00Z')
The flow would run successfully as below:

Regards,
Alice Zhang