We can measure the time and space of an event in a coordinate system by ploting an index line perpindicular to the axis in question and through the event. The interval between two events is given by the distance between the indices. Here we have an example of this technique applied to a tachionic situation. When you look at the time axis you will notice that both legs of the trip have a time flow in the same direction as home, but at different rates. But the two legs have time flow opposed to each other.
Home is at tachion speed relitive to a tachion.
The state of being a tachion is relitive.