If a contract is written such that a payment is due on day x does that mean it is due prior to that day or any time before 11:59pm on day x? For example I've heard of a property management company that stipulates "rent is due on the first of each month" but if the rent is not received by the time the office has opened on that day tenant gets a late rent charge and a nasty phone call at 9am.
In general, the words "due on" take on their ordinary meaning - that is, that the performance of a contract is required on or before the date specified.
If it is not specified in the contract, a court will look to what is reasonable in the circumstances. For instance, if payment is due to a company that trades 24 hours a day including the day payment was due, then payment up until midnight may be found acceptable.
However, if it is not known to the person making the payment that the business has such extended hours, and especially if it is known that the business has normal hours, then payment after close of business would likely not be reasonable.
To your question, and having reviewed some Canadian residential rent legislation, although none define due on specifically - and I would not expect them to do so - most have clauses which provide for termination, and they only refer to serving notice either a certain number of days after rent falls due, or the day after rent falls due.
This would tend to support my assertion that rent is payable on or before the specified date, unless the contract says otherwise, in line with the above considerations.
NB: While rent is usually payable in advance, this simply means that you are paying for the occupancy during that follows, or substantially follows, rather than the period that has elapsed.
If a contract has a due date it means on or before that day. If it does not specify a time it means before 11:59:59.99PM on that day.
For you real estate manager rents are usually payable in advance. For a monthly rent this means it must be paid before the month starts. The due date is therefore not the 1st but the 28th, 29th, 30th or 31st (as the case may be) of the month before and the manager is being vigilant in the owners interests.
The rent is due by or on the due date because the property management agency is forbidden by law to require rent to be paid if the current rent has not yet been expensed. No time - definitely means by 23:59:99 on due date. If electronically paid, the rent is deemed paid as soon as the submit button is depressed, and this irrespective of when the rent amount appears in the property managment's trust account and subsequent information system's rent ledger.