You can use Caché Studio > New Property wizard to get correct definition of "collection" property, select options - "A collection of Type" = array/list , "Containing elements of type" = %Date.

It then creates a definition:

You can set and access array data as follows:

ClassMethod Test()
{
 // Get values
 Set obj=##class( collect.arraylist).%New()
 Do obj.DOB.SetAt($h,"key1")
 Do obj.DOB.SetAt($h-1,"key2")
 Do obj.DOB.SetAt($h-2,"key3")
 
 // Get values
 Write "Count: ",obj.DOB.Count(),!
 Set key=""
 Set date=obj.DOB.GetNext(.key)
 IF key'="" Write "Data:",!
 while (key'="") { 
   Write key,?5,$zd(date),!
   Set date=obj.DOB.GetNext(.key)
 }
 
 Write !,"Data at key2: ",$zd(obj.DOB.GetAt("key2"))
 Quit
}