An assured shorthold tenancy is either for a specific (fixed-term) or periodic tenancy, which continues on a weekly or monthly basis.
Times, Sunday Times (2018)
The two indices often differ on a monthly basis.
Times, Sunday Times (2016)
On a monthly basis they were 1.1 per cent higher as the imbalance between the chronic lack of homes for sale and soaring demand continued to intensify.
Times, Sunday Times (2015)