This would make developing a science fiction or fantasy story a lot less awkward

What about entering birthday dates? If the calendar field would let you enter only numbers, could you live with "-7330" (days or "time units") as birthday?

First annotation for the following comment: I'm working with storybook for a few months now and I might not know all features by now, so please excuse if any suggestion was already mentioned or is already implemented! ;) (perhaps my bad english, too)

I agree with Tim, it would be very useful for fantasy stories.

I'm not sure if I get the point with the birth dates right, I would add these to the "character" (#4), but in a easy day-by-day-system, I can live with it (at least for my current project).
For the calendar topic itself: There are figures, there are places, there are tags, there are items - but I am missing something like "story outline" where I can enter details for world (perhaps comes with #12?), society, ... and this would be a good place for something like calendars or for choosing a "time system", i.e. some predefined like the gregorian (julian? chinese?) which you can choose or and which is used over the whole project for calculating ages...
... or define an own one, but this could be a bit more difficult.

Just a thought...

For any calendar system, you want a way to create cycles, and cycles within cycles. You might be on a world similar to ours with longer or shorter year, you might have a moon with longer or short month, or that moon might be tidally locked so one side always faces the same way toward its planet;

Everything would be handled easily with a way to create cycles (similar to category) and units that can be labelled, tell how many units in a cycle, and be able to relate cycles to other subcycles.

For our familiar units, define
Cycle of Year, and units of Day (365), add exception every 4 years to account for leap years, so array of lengths of each Year (365, 365, 365, 366)
Cycle of Year and units of Months (12), labelled by name (Jan, Feb, March, April, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec)
Cycle of Month and units of Days, add number of days in each month (31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)
Cycle of Year and units of Weeks (52), weeks not named but numbered; account for fact that only adds to 364 days. (1, 2, ... 52)
Cycle of Week and units of Day. (7) Add labels for each day, (Mon, Tues, Wed, Thurs, Fri, Sat, Sun)

So you need to define a Cycle, a unit, an array of labels, an array of numbers, and handle the logic of how they relate.

Logic for the above is:
Day is smallest unit.
Week is fixed length.
Month varies in length, depend on the labelled name.
Year is fixed length in months.
Year is varies according to if it is a leap year.
Every date has a label (weekday), week, month, year assigned to it.

You can then do something like:

For the planet AwesomePlanet, let's say:
Cycle of "Cycle" with 600 units of "Risings", leap year every 7 year, so numbered by length (600, 600, 600, 600, 600, 600, 601)
Cycle of "Cycle" with 10 "Voyages", labelled by name (alpha, beta, gamma, delta, epsilon, rho, cho, mo, fo, ho)
Cycle of "Voyages" with 6 units labelled "Journeys", labelled by name (ex. names of their heroes) (prometheus, hercules, perseus, sinbad, jason, thesius)
Cycle of "Journeys" with 10 units of "Risings" each, labelled by name (1st Rising, 2nd Rising, 3rd Rising, etc.)

Logic is:
Rising is smallest unit.
Journey is fixed length, no exception
Voyage is fixed length of both risings and journeys, no exception
Cycle varies according to if it is a leap year.
Every Rising has a label, journey, voyage and cycle assigned to it. (ex: 2nd Rising of Perseus, Alpha Voyage, 4000th cycle.)

Many thanks for your example!

Hi, I remember requesting this so I'm glad it's being looked into.

The example by spacechampion is essentially what I needed for a fantasy novel. Customizable names for days, weeks, months, years. Customizable lengths for each of them.

I guess just a field where you enter the name that would replace (mon, tues etc) as well as the number of days in a 'week' would be perfect. The option to just ignore one of the fields. Say for example that people in the novel don't think about weeks but only months. That would also help for any sci-fi books where cycles would be vastly different due to potentially not being on a planet with regular cycles.