[O] How to calculate all time durations in a file automatically?

[Martin Schroeder]

I have imported a bunch of log lines with CLOCK periods from another
program into an org file but I have forgotten to calculate periods. So
my time lines look like this:

CLOCK: [2019-03-27 Wed 19:30]--[2019-03-27 Wed 21:30]

Instead of this:

CLOCK: [2019-03-27 Wed 19:30]--[2019-03-27 Wed 21:30] =>  2:00

I can now no longer check in on any task until I fix all these ranges
in all of my agenda files. So I have to go to each one and do C-c C-c.

Surely there has to be a better way but in my searching with a giant
flashlight I was not able to find it yet. Can anybody point me in the
right direction?