There's no defined way to describe epoch in the spec. At the moment, there's only one way to describe a timestamp.
You can use the 'format' field with whichever value, as it's an open field. Just keep in mind tooling will not automatically know what it means. Some tools may offer extensibility to support custom formats.
In OpenAPI 3, there's a process of building a registry of additional formats. You can find more information about it here.