calendar notification time wrong

THIS IS NOT A TIMEZONE PROBLEM.

On my Macbook (running High Sierra 10.13.3) I get calendar meeting or event notifications (banner).

When I get a half-hour alert beforehand, this arrives on-time. However, the banner says 'now' in the top right-hand-side. Same is true if it a ten-minute warning:

the meeting is for 9:30, but the time is 9:20 -- this is my ten-minute warning! Why does it say 'now'? In fact the time 'counts down', so if I look in the notifications column two minutes later it will say '2m ago' on the message, but the meeting it is talking about hasn't started yet!

When I get a calendar alert for a day (like a birthday), I get it the day before (as requested) but the banner says 'Today' in the top right-hand-side.

Here is an example:

but in calendar:

These notifications seem to say when the alert is raised, which is singularly unhelpful, and very misleading.

This behaviour doesn't happen for (the same) calendar notifications that appear on my iPhone: the banner time says (correctly) when the event is to happen, and how long I have to go.

This looks like a bug.