He travelled during Ramadaan to a country where the people started Ramadaan on a different day

What matters when a person started fasting in one country and
finished fasting in a country to which he travelled, is that if the total
number of days that he fasted was twenty-eight, then he must make up one
day, because the lunar month cannot be less than 29 days. If he has already
fasted thirty days but the people in the country to which he has travelled
still have one more day to fast, for example, then he must fast with them so
that he may break the fast with them on the day of Eid, and offer the Eid
prayer with them.