Your contract with Uniplaces will always have a start date and an end date: this usually decides how much you will be paying - especially on the months you move in and move out. There are four kinds of contract, make sure you check out which one applies to the places that you're interested in:
Fortnightly - On a fortnightly contract, you are charged for every fortnight booked. So if you move in at any time of the second fortnight of the month, you will be charged for half a month, and not for the entire month.
Monthly - In a monthly contract, your are always charged for a whole month - regardless of how many days of that month you stay. So if you move in on the 20th, you still have to pay a whole month. Because of this, the contract start date will always be the first day of the month.
Fixed - In a fixed contract, the landlord has set a fixed start and end date for the contract, and you will always have to pay for this full period. A fixed contract can cover six months, for example, and you have to pay this regardless of whether you stay the full period or not.
Daily - In a daily contract the rent paid is calculated in proportion to the days of your stay.
You can find the contract type in the listing page: check out the Rental Conditions section, under the place details.