API: possible completions

when you open a Terminal and type

/sy clock set time-zone-name=

, then ‘Tab’ button - you can see

Africa/Djibouti                 Asia/Jerusalem        
Africa/Douala                   Asia/Kabul            
Africa/El_Aaiun                 Asia/Kamchatka        
Africa/Freetown                 Asia/Karachi          
-- Press TAB key again to see more possible completions

is there a possibility to read this timezones via API?

these are not currently accessible through API. These values maybe will be added in the future but that is indefinite.