Make skyplot time-start command line arg more intuitive

[bgottula]

I'm trying to use skyplot and the next pass isn't the one I'm interested in, it's too hard / confusing to figure out what I should pass to the --time-start argument to get what I want. I think part of the problem is that doing mental math to figure out a UTC time is too difficult. Furthermore, when only a time is provided, it's not always clear what date will be used (today? tomorrow? yesterday?).