Current version of the spec defines epoch as:
Epoch: Description: The Discovery Endpoint's epoch value for this Service Entry. This specification does not mandate any particular semantic meaning to the value used. For example, implementations are free to use a value that represents a timestamp or could choose to simply use a monotonically increasing number. The only requirement is that the value MUST always increase each time the Service Entry is updated. This allows for a quick integer comparision to determine which version of this Service Entry is the latest - meaning, the one with the larger integer value.