Add more Date and Time helper methods

[earlgaspard]

It would be nice for UtiliKit to provide more helper methods for dealing with dates and times.

[tylermilner]

Do we have any feedback on what kind of date/time functionality is currently missing? I wouldn't want us to spend time build something just for the sake of "adding more". If/when we need some enhancements, we might also want to take a look at some popular third party date/time libraries:

• https://github.com/davedelong/time
• https://github.com/malcommac/SwiftDate
• https://github.com/MatthewYork/DateTools