I think you’re right. The issue is probably with the fact your timestamp is a numeric value and you need to convert it first to a proper date datatype. I think you can do that with the parseDate() function. Once you get the number into a date data type you can use addDays() on the resulting value
addDays(parsedate(<your timestamp>),-1) should return a proper date data type.
Now the Hubspot filtering expectation is a whole nother matter. I think the GTE operator may expect a simple date without the time component like
2010-01-01
You should experiment to see without any variables what sort of format actually does work by hard coding the value part of the filter for the Hubspot Search module. See what format works and see if you can get your data into this format.