RQG PDF page 28:
"Some events are only important to certain Homelands. If your homeland is listed as participating, you must roll or pick a result. If your Homeland is not listed in the Participating Homelands, you can skip the event if you choose. You can still choose to roll, even if your homeland is not listed as participating. History has a way of affecting everyone, no matter where you are from."
If you really really want your Old Tarsh or Praxian character's grandparent to join in on the fun in 1597, I assume you are not meant to pick any result higher than 15 (20-5), and if you roll d20, apply that listed -5 modifier.