The date you are adding is one which is in the list of years for which there are no occurrences
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Clearly this is inconsistent: you cannot tell Q++Studio (at the bottom) that the selected holiday does not occur in 2020 (for example) and then add the date of March 12, 2020 (at the top).
This message is there to let you decide which of the 2 options to choose.
If you click on Yes, then the specific date will be added to the top list and the year of that date will be removed from the bottom list.
If you click Cancel then nothing will happen: the specific date will not be added to the top list.
Topic 173920, last updated on 18-Apr-2020