Tweaks to Gigasecond Exercise

[elfenlaid]

During my experience reviewing Gigasecond exercise, I often notice how hard it's to work with custom date formats, time zones, and calendars.

I suggest to:

• Make input dates compatible with ISO8601DateFormatter.
  • They are 'Z' postfix away from property confirming to ISO8601 anyway
  • That will remove the necessity of dealing with custom time formats and nurture best practices of dealing with time formats (use existing formats)
• Explicitly mention ISO8601DateFormatter and the necessity of setting its .timeZone in the exercise's notes