Does saying ‘due by’ mean last day to do it before that date or on that date too?

I've always understood 'due by X date' to mean it's fine to turn something in or pay up to and including that date. You have a legitimate reason to be upset with your university.

I'd be on your side here. "Due by Feb 1st" means Feb 1st is the last "on time" day for payment, not the first "late" day. We could quibble about whether that means during business hours, or any time before midnight, but if they meant that payment must be made before the end of January, then they should have said so.

I agree that this is unfair. I am in the U.S. and have never seen such a usage.

When you say that you were sanctioned, do you mean that you were charged a penalty for late payment?

Can I ask where you are located?

That sounds unfair to me. If you told me that my homework was due tomorrow, that doesn't mean I give it to you today.

It means on that day, not the day before. There is no room for interpretation. This is the nationally understood meaning by everyone who does business.

If something is "due by" feb 1st, then feb 1st is the "due date". You have at least until the end of the business day (sometimes until midnight) to submit payment on the due date.

The university may try to get around this by putting a different definition of "due date" into the contract, but it is something that would need to be pointed out to you clearly. In US law, unusual terms like this usually need to be clearly spelled out. If that isn't done, then there is a good chance a judge will nullify that portion of the contract. This would require you to go to court, and I'm not sure how that works if you are not a US citizen.

I always took "due by 1st Feb" to mean you have until end of day on the 1st Feb to hand it in. That's how it was when I was at university.

Unfortunately it's inherently unclear to say "due by" unless you give a specific time like "Due by 1pm on Feb 1." It can be argued either way, but an institution like a school needs to be more clear.

My understanding is that if the payment is received ON or before the due date it’s on time. If it is received after the due date then it is late.

Due by 1st of Feb means that the funds must be there by the end of the business day on the 1st of Feb. There is no room for misinterpretation. Another way to express the same thing would be on or before (or no later than) the 1st of Feb. Is the university in an English-speaking country? If not, then perhaps you need to point out their mistake.

Strictly speaking if it says “due by” the subject needs to be completed before that day arrives.

If someone said to you I want this done by 1pm, you wouldn’t think you had until 13:59 would you.

Now that is the strict definition of the usage, I can attest to having found a large number of people who think that if something says “by February 1st” they can still complete the action on February 1st. And I have found varying responses through education and business. Some people are fine waiting till the end of that day and some people aren’t.

Regardless, if for some reason you ever have to take this argument to court you would lose as the definition is pretty clear. Once the clock turns 00:00 on February 1st if the item isn’t completed you will technically be late.

The best option is to always assume the worst possible outcome and work within those parameters.