An array which defines an integer range within which the value must be. Keys:
Defines the earliest date.
Defines the latest date.
It is allowed to specify only one of both of them.
In this example the upper limit is set to the last day in year 2020 while the lowest possible value is set to the date of 2014:
'range' => [ 'upper' => gmmktime(23, 59, 59, 12, 31, 2020), 'lower' => gmmktime(0, 0, 0, 1, 1, 2014), ],