$Date vs $$Date

$Date - is an Internal method (i.e. field) of the VOUCHER Object.
$$Date - is a function that accepts a string parameter and returns date value.

Example:-

Select $Date,$VoucherTypeName,$Amount from RTSAllVouchers
where $Date = $$Date: '01-04-2007'

The above SQL-SELECT statement returns all Voucher records where the Date is '01-04-2007'. Please note that condition $Date = $$Date: '01-04-2007' wherein the 1st $Date refers to the Date field whereas the 2nd $$Date is a function.

Remarks:-
1) RTSAllVouchers is a user-defined Collection and the related TDL which defines it must be specified in the Tally.ini. Click here for more information on RTSAllVouchers.
2) RTSlink DLL v1.8 or higher supports SQL-SELECT. Click here on more information on how to use SQL-SELECT statement.