Per forum post: https://www.zabbix.com/forum/showthread.php?t=60381
It would be very helpful if we could have aggregate and calculated items compute their retroactive historical values based on a user-defined period.

For example, I have host group "Big Servers". All hosts within this group have been gathering data for the item Since 1/1/2018:

system.cpu.load[percpu,avg15]

On 1/10/2018 I create this item on a dummy host:

grpavg["Big Servers","system.cpu.load[percpu,avg15]",last,0]

The history of the aggregate item only goes as far back as when it was created (1/10/2018) even though technically we have the raw data required to compute the value of this item as far back as 1/1/2018.