The simple answer is HSQLDB 1.8 is an old version and does not support some advanced functions. HSQLDB 2.3.x supports a full range of functions to convert between different representations.
With HSQLDB 1.8 it is possible to create your own function in Java to convert the millisecond value of a Timestamp to a java.sql.Timestamp or java.sql.Date
Something like this in a class that you create:
static java.sql.Date millisToDate(long millis) {
return new java.sql.Date(millis);
}
If you then include this class on the classpath used for running the database, you can call the function in your SELECT statement like this:
SELECT ENTRYID, "myclass.millisToDate"(REVOCATIONDATE) FROM ENTRY
Your result example will look like this:
SELECT MIN(ENTRYID), COUNT(ENTRYID), "myclass.millisToDate"(REVOCATIONDATE) FROM ENTRY
GROUP BY "myclass.millisToDate"(REVOCATIONDATE)
I haven't actually tried any of the above suggestions, but they should work, perhaps with some alteration.