Fictional calendar/constellations?

[parduz]

Is it possible to set Celestia working with an alternate calendar?
I mean: if i set up a planet with a day 28h long and with a year of 310 days, could i set and show the date/time related to that planet?

And
There's any way to name and draw fictional constellations?

[selden]

Is it possible to set Celestia working with an alternate calendar?
I mean: if i set up a planet with a day 28h long and with a year of 310 days, could i set and show the date/time related to that planet?

You can't change Celestia's internal dating system, which is based on TDB. It depends on the precision and accuracy of TDB in order to be able to plot accurately the courses of real space probes. However, you could write a Lua function to display the information you want.

And
There's any way to name and draw fictional constellations?

Yes. You can replace Celestia's files data/asterisms.dat to change the constellation figures, data/boundaries.dat to change their boundaries, (either edit the files or change the entries AsterismsFile and BoundariesFile in celestia.cfg). Unfortunately, changing the constellation names is more difficult, since they're built into the Locale code. However, you could define deep space objects in a .DSC catalog with appropriate names.