science fiction - How can I convey dates (with ordering) for events happening under different dating systems, e.g. a Sci-Fi story spread over the galaxy?

So I want to write a Sci-Fi where humans have spread across the solar system and into the galaxy. There are multiple stories going across at several different times. E.g.:

1. Aliens attack Pluto


2. 3 weeks later, aliens arrive at Mars


3. Flashback scene to a few years ago, when Earthlings missed sign of invasion, leading to destruction of Earth.

Currently, I'm using Earth dates, something like: July 28 (Earth date), aliens attack Pluto, August 20 move to Mars etc. But people on Pluto wouldn't use the Earth calendar; they would use their own system. When those people talk about dates, what should they say? How can I convey to my readers the dates of these various events (and the relationships among them) when the calendar systems used by the characters would be different?

PS: I don't want something confusing like the Stardate system. I don't want to be explaining to my reader how my calendar works.

Answer

I can think of a few ways:

1) Cheat. This was how Tolkien did it, so you'd be in good company. He just listed somewhere in the appendices that "Year 5798 by Gondor's calendar = 144 Shire Reckoning" and let the readers do the math.

2) Make the characters work out a solution. If you have characters on Terra and characters on Pluto who meet, they're going to have to work out some way of syncing their calendars.

3) Meta. At the beginning of a chapter, just announce as part of the header what the date is:

July 20, 2056 Terran Time

12 Fizzbinth of Shar, Plutonian Time

If you equate them at the beginning of the chapter, it will be clear that the two dates are the same, and then you can advance them separately (or together) as needed.

4) Reset the calendars. Call it 1 A.I., Year one after invasion.

At some point, unless all your characters are using a calendar which your reader understands, you are going to have to do some amount of explaining.