how to select datas from database

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C#

i used a query to get data from database depending upon the datetime property

the query was like

string sql = "select * from   tblVehicle where CAST(floor( CAST( Vehentrytime AS FLOAT ) )AS DATETIME) = '" + myDate + "" + "'";

here i got the string for myDate as

string myDate = System.DateTime.Now.ToString("MM/dd/yyyy");

now i want to get all records selecting datas of a month or may be year also

i tried like

string myDate = System.DateTime.Now.ToString("MM/yyyy");

but encountered some error saying conversion failed
since my system time is in format mm/dd/yyyy how can i obtained my desired values
with regards bishnu