If you already know the value in hours, then you can compare any given date to a 'base date', i.e.
DateDiff ("h", BaseDate, YourDate)
You could also get a bit of astronomy software that will tell you the phase of the moon, which you do need if you're a serious star-gazer. (Mars looks a lot better with the moon below the horizon, as is currently the case from England.) There's probably free software that would tell you, and you could generate an Excel table and feed it into Crystal.
Madawc Williams
East Anglia, Great Britain