Investigate use of Java 8 time libraries

[ssoper]

Consider replacing use of GregorianCalendar with modern classes like ZonedDateTime

[ssoper]

Another reason to move off of GregorianCalendar? No more pesky one-off month value.

[ssoper]

Related #99. Should look to move to ZonedDateTime for all dates and use America/New_York as the default timezone (since that’s where the NYSE is).