api filtering by date

We have a vendor that is pulling tickets from SWSD to their ticketing system (dont ask why our environment has 2 ticketing systems...) and asked me the following question that I cannot answer

"I have successfully filtered the call to only get Engineering/EVS Incidents. My issue is filtering the results based on their created date. I can filter with 'created_at=9-9-2021' but if I try something like 'created_at>9-8-2021' it fails. I am also not if the call accepts timestamps, I can't get that to work either. I would like to do something like 'created_at>{last runtime timestamp}'. "

Is something like this possible?  Thanks for the help, I am not good with APIs.