How is time in TimeHeadwayCondition calculated?

We have TimeHeadwayCondition which mentions calulated time but it doesn't explain how is this time calculated. Headway in OSC1.0 means "shorter" distance:

The precise definition varies depending on the application, but it is most commonly measured as the distance from the tip (front end) of one vehicle to the tip of the next one behind it. It can be expressed as the distance between vehicles, or as time it will take for the trailing vehicle to cover that distance. A "shorter" headway signifies closer spacing between the vehicles.