In the Designer we are able to choose from a list of date/time formats, however there is no option to select UNIX timestamp:
Also check the reference docs: (Date/time field | Webflow University)
In my case I wanted to add a countdown to show when different events were to happen. The problem I encounter was that I needed to get the Date string from html and then transform it into a Date with javascript. By doing this each date was created based on the client’s time and not the actual time timezone of the project in Webflow. So I had to manually add the zone to the string to be able to convert it correctly. But then I came with another issue, that in Australia we have Daylight savings and then I had to add another layer to use the right timezone :$.
If we had the option to select a UNIX timestamp to be selected from the date/time dropdown, then we would be able to easily convert that into a Date using javascript with no issues with timezone as UNIX timestamp are timezone independent (Unix time - Wikipedia).
Thanks!