The UpdateExpirationForHIT
operation allows you update the expiration
time of a HIT. If you update it to a time in the past, the HIT will be
immediately expired.
mturk_update_expiration_for_hit(HITId, ExpireAt)
[required] The HIT to update.
[required] The date and time at which you want the HIT to expire
svc$update_expiration_for_hit( HITId = "string", ExpireAt = as.POSIXct( "2015-01-01" ) )