The selected date does not match the Gregorian calendar.

It's easy to explain. There is no standard version of Gregorian calendar. When we choose a day, we should not only look at our family's favorite god, but also look at whether there is a rush or not, whether it is the ecliptic, and then avoid the three evil days, the red-billed Suzaku Day and so on.

And every school is different, and the inheritance is different. Take feng shui as an example. There is a difference between situation and regulating qi. There are Yang Gong in the Eight-character Courtyard and so on.

Besides, it is not impossible to do it even on a bad day. If you have to get married on the White Tiger Festival, you need a unicorn sign. If a family must get married on the moon, it is feasible to find the combination of year, day and month or year, day and month.