If a user enters an event time as 7:30, all we tell them is "This is not a valid time". That makes sense for something like "fidhfh" or "7:66", but if we detect a valid 12h format time, we should probably give a more useful error, like "The time field should use the 24h format and two digits for the hour (08:15 or 20:15 instead of 8:15)."