If you are trying to do date manipulation have a look at the Add Date Function calls for microflows.
It looks like you want the date 13 months before the end of the current month, so you would probably want something like
addMonths([%EndOfCurrentMonth%], -13)
Hope this helps
Except addmonths isn’t available in OQL from what I understand, and I’m trying to replicate my OQL logic so I can verify its result in a microflow (since OQL won’t let me display this value).