On my S7 Edge, the option for the clock is found here....SETTINGS -> DISPLAY -> "Always on Display" (Mine is set for Clock, All day) as well as "Night Clock" (Mine is set for 7:00 PM to 7:00 AM).
The S8 might be a little different, but, it could be same.
Making sure you can check your time from your lock screen is important, VJC9890. The Galaxy S8 moved the Always on Display settings. You can locate this setting under Settings > Lock Screen and Security > Always On Display. Hopefully this helps.
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