My parents came from US on a Visitor Visa vi a land border. they were asked about duration of stay nad they replied 4 months . The officer at Port of Entry asked them to leave in 4 months and said that he noted that down and will check after 4 months. However I do not see any stamp on their Passports for the exit date. can they stay for 6 months given there is no stamp on the passport per guidelines or how do we know the exact date that they can stay for?