This sounds very close to default QlikView functionality anyway. The dates would not be selected, but only the associated ones would be active. Why do they actually need to be selected?
The above depends on the data model being correct, eg. all the different parts of the date being in the same table a the date itself - or in a joined calendar table. Is this the case?
If you do need to apply the selection, then Select Possible is probably the way to go - as it would be less coding than Select In Field.
The Year, Month, Day are just broken down from the Date field. The reason I want to do this is, I have a few charts that plot the past 30 days based on Date. My date selections are broken down by Year Month and Day for easy access by the user. However, when the user clicks on a specific date using the Year Month Day, the chart no longer shows the past 30 days of data. When I tried to select the actual date from the Date field, the chart works properly.