Why is rel_date_diff so crippled?

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We've several hundred teams all with different schedules and hours of operations. Many of the managers want us to calculate “Time to resolve” excluding Nights and weekends. But due to the crazy restrictions on rel_date_diff I can’t even time_format the field, much less do an average. And because we’ve multiple teams with multiple shifts, the entire SLA system is a no-go for us, because it applies the SLA to the contact/org instead of the profile/queue/group.

 

I found that someone had suggested improvements in the idea lab 4 years ago but it’s never been fix. Has anyone found a way around this?