The date a student joined your school is required for the census. This date is taken from the enrolment details on their profile.
However, you aren't able to use this field in our in-built reporting pages, such as for defining what contextual data shows in marksheets.
To report on how many children joined your school between two dates, you can use the Joiners page using these instructions: See students that have joined my school
It is not possible for Arbor to automatically calculate the length of time spent in the school since the entry date you would need to download the data into Excel to do this.