isnt what you have said the same thing just worded differently?

shapes:

 

if it say offer ends 23/06/2020 does that mean offer is valid till that date or that 23/06/2020 is the last day offer is valid?

 

Ah, one of the many annoyances of advertising. If I were writing, till would mean prior to, and midnight 23/06/2020 would mean the offer expired at the start of 23/06/2020, i.e. 00:00 23/06.

On the other hand, common usage is just plain wrong, and I'd expect complaints from customers and probably regulatory authorities.

throbb: Generally it is inclusive of the date mentioned.

I would agree with that.

Also it is a bit like Due Date for a bill... You have the whole day specified to get the payment made.