Two similar but independent features for the 'Statistics' tab to add custom date ranges:

1. Allow the user to add new date ranges to the preset dropdown box.
   These can be 'fixed' ranges with a start/end date set by the user and a label.
   Additionally it would be great if the user could add 'relative' dates like "This week", etc.
   This would be helpful for countries with financial years end partway through the year (Eg Australia)
   It would also be useful for custom development 'Sprint' periods.

2. Allow each 'statistics tab' to have its own date range that can be saved.
   Eg it would then be possible to create a 'tab' for "Day duration - this week", "Top Tags - this month", etc without having to adjust the date range each time