Here is the screenshot from the Apple Calendar event quick preview:

Why it is not a
Alert at THE time of event
or
Alert at time of THE event
or even
Alert at THE time of THE event
I suppose there are definitely no place for "A" article in reason the subject of the question is specific and known - we know what is the time and the event.
But why there are no definite article?
And is it correct to say "Alert at time of event" in conversation?