... Must be uploaded by 11:59 p.m. (GMT) on April 12, 2016.
What does "by" mean in this context? Does that mean "before"? So the deadline is on April 12 at 11.59 p.m. GMT?
And 11:59 p.m. = 23:59 (24h), right?
English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.Sign up to join this community
Even to native English speakers, "by" is ambiguously either inclusive, or not inclusive of the date/time being mentioned. For example, if you say "have it on my desk by Friday", some English speakers will interpret that as including Friday, and some (like me) will not. That's one reason why people often try to make it more specific, as they have done here, so that the ambiguity is not as important.
Judging by the very specific time, I'd interpret this as an attempt to make clear that the data must be uploaded before April 13 2016 starts. 11:59pm is 23:59 on a 24 hour clock.
Personally, when I hear or read 'by' used in a similar context (in the question), my interpretation is that the person who delivered the message means:
"Do this, because by that specified time, I am expecting that it is done." (Of course the person won't tell you whether or not that task would be checked by that time)
In this case, uploading, make it sure that if there's a lag in the system, the file must be 'visible' and 'accessible' when the person checks it during the specified time.
Regarding your interpretation on 11:59 PM - it is correct. The deadline is on April 12, at 23:59:XX.