This is the script I ended up using. There is probably a more elegant way of doing the same thing, but this works well enough for my purposes:
New Column( "start.month", Numeric, Continuous, Format( "Best", 10 ), Formula( Month( :start.date ) ) );
New Column( "start.year", Numeric, Continuous, Format( "Best", 10 ), Formula( Year( :start.date ) ) );
New Column("start.date.rev", format("m/y"),formula(Date MDY( :start.month, 1, :start.year)));
for each row(
If(IsMissing(admin.date),admin.date=start.date.rev);
If(IsMissing(admin.year),admin.year=start.year);
);