I'm using LineageOS so I have the AOSP Phone app. If Graphene uses the same, then you can actually determine the exact time of call, but you need to jump through a couple of hoops: Go to "Call History" from the three dots then click on the call of interest. You'll see a button called "Call Details". Then you can see the exact time.

Can confirm the same works on Graphene. Also works directly from the "Recents" list.