Remember that each Bahá’í day ends and a new one begins at sunset. This is probable the most “reality-shaking” experience of the Badí calendar (Badí is the name of the calendar itself as revealed by the Báb).
An event that begins after sunset and ends before the next sunset; is on the same Bahá’í date. The same event will cross midnight and thus belong to two different dates in the system-calendar. The app therefore shows the system-date before each event, to easier see their relation in time
In context of a e.g. work-meeting, just look at the system-dates to follow the same reference as your colleagues… You’ll still notice the Bahá’í date, and gradually grow consciousness of it’s dates and progress as well.
Note: References to the “system-calendar” practically means the “Gregorian” calendar. But different calendars are in use around the world, so I therefore choose to refer to it as “system-calendar”.