By default, Samsung devices display two clocks on the lock screen when the device is in a roaming network (in another country). Both local time (roaming clock) and home time are displayed. The roaming clock can be turned on/off in the lock screen settings.

Note: If Roaming Clock is enabled, the Clock on the lock screen cannot be personalized.

Android 10 (One UI 2.0)
Step 1:  Open the  Settings
Step 2:  Tap on  Lock screen
Step 3:  Enable or disable  Roaming clock
  1. Open the Settings
  2. Tap on Lock screen
  3. Enable or disable Roaming clock

Samsung Instructions