Need help with COQL date operators

Need help with COQL date operators

How do I use a variable in the SELECT statement to compare a field value to a date variable

Renewal_Date is a CRM date-time data type.

This generates a data-type error:

aDate = today;

"{\"select_query\" : \"select Last_Name, First_Name from Contacts where Renewal_Date >= aDate limit 200\"}"