Store next run NSDate in preferences

[kylekingcdn]

This should be used to handle consistent scheduling across helper app restarts etc.

[kylekingcdn]

Further clarification on updating the NextExportAt value.

Update triggers:

• When the helper app launches
• When any of the following values change:
  • scheduleEnabled
  • scheduleInterval
  • lastExportAt

Updated values:

• If lastExportAt is nil or time elapsed since lastExportAt is greater than scheduleInterval:
  • nextExportAt = currentDate + scheduleInterval
• If time elapsed since lastExportAt is less than scheduleInterval:
  • nextExportAt = lastExportAt + scheduleInterval