Alright, I think it will be helpful to post the queries for all three databases.
For MSSQL,
DATEDIFF(day, DATEADD(s, wos.LAST_TECH_UPDATE/1000, '01-01-1970 00:00:00'), getDate()) > 20
For Pgsql,
extract(epoch from(now()::TIMESTAMP -to_timestamp(wos.LAST_TECH_UPDATE/1000)::TIMESTAMP))/3600/24 >20
For Mysql,
SELECT wo.WORKORDERID AS "Request ID" FROM WorkOrder wo LEFT JOIN WorkOrderStates wos ON wo.WORKORDERID=wos.WORKORDERID LEFT JOIN StatusDefinition std ON wos.STATUSID=std.STATUSID WHERE std.STATUSNAME = 'Waiting for User' AND wo.ISPARENT='1' and datediff(curdate(),from_unixtime(wos.LAST_TECH_UPDATE)) > 20