You may have recorded the Date for Entering the UK for international students coming from oversees countries in your previous MIS. For example, in Cloud School this was called UK Date of Entry.
This information is not required for the census. Arbor doesn't have a built-in field to store this data, so we won't have migrated it from your previous MIS.
If you want to store this, you can set up a date-type User Defined Field.
It is not possible for Arbor to automatically calculate the length of time spent in the UK since the entry date.