In Oracle, adding X to a DATE will return you a DATE X days later.
If ESTIMATEDENDTIME is milliseconds since Epoch then you could do
DATE '1970-01-01' + ( 1 / 24 / 60 / 60 / 1000) * ESTIMATEDENDTIME
and then use to_char to achieve the correct format of the resulting date. e.g:
SELECT
captureid
, startdate
, enddate
, state
, estimatedendtime
, DATE '1970-01-01' + ( 1 / 24 / 60 / 60 / 1000) * estimatedendtime AS estimatedenddate
FROM recording