DateTime to string problem???

[r4m1r3z]

DateTime lastLogon=row.lookup_by_name("LogonTime");
query<<"SELECT * FROM Posts WHERE CreationTime>%0q";
query.parse();

Template query is set, so when i execute it it works just fine (in this case):
result=query.store(SQLString("2005-03-03 12:00:00"));

But that's not what i want, I don't want to pass date & time as a string, I wanna pass it as DateTime object lastLogon but don't know how to convert it...

result=query.store(SQLString(lastLogon)); //this doesn't work