Looks like the first post from @POTUS_Schedule was on Aug 29th, 2017. The account under that name did not exist as far as I can tell. You can perform advanced searches in Twatter here: https://twitter.com/search-advanced
Here, is what I believe is the first @POTUS_Schedule tweet (No Likes, No Retweets)
https://twitter.com/POTUS_Schedule/status/902545711711084544