Our story leads to places on many different planets, and all of them take different times to go around the stars that they use as suns. This means they all have different lengths of year, which isn’t too much of a problem. It’s quite easy to make up a calendar to fit the numbers. For example Jane’s home planet, Mercia, has a year which is just over 581 days. The calendar they use divides this into 83 weeks of seven days, and 21 months of which 20 have 28 days and one has 21 days, or 22 in the occasional leap year.
The real problem is telling someone the date of an event, or working out how old somebody is, when the calendar they are using looks absolutely nothing like yours.
Jane sat back in the seat, calculating in her head. Mercia, her home planet, had circled the F-type star that it used for a sun a little over thirteen times in Jane’s lifetime. But converting her date of birth to the atomic time scale used between the stars, then allowing for tidal drag, and the effect on the Earth’s rotation of eight centuries of intermittent nuclear bombardments, today was Tuesday the 21st of May 2830 – Jane’s twenty-first birthday. And, if God was still on speaking terms with her, she’d like to ask for just one present – some way of stopping Arthur before anyone else had to die.
So how do we make that calculation?
The trick is to have a clock and an almost-Gregorian calendar that gives the time and date on a “pretend Earth”. Seconds, minutes and hours have the lengths we know and love, but there are no leap seconds, and the pattern of leap years we use now is kept without any attempt to correct it. This operates in parallel with the planetary calendars. Every day on a planetary calendar has the date according to the “pretend Earth”, or Galactic Co-ordinated Time to give it its proper name, printed next to it. Of course because of the difference in the lengths of days sometimes a GCT date is printed on two adjoining planetary days, or two GCT dates are printed on a planetary day. For legal purposes, or fun things like knowing when birthdays are, you use the GCT date. For things that depend on the seasons, such as farming and summer holidays, the local planetary date works.