SQL syntax for "week-to-date"

I need to find out total qty sold "week to date" on a SQL query, something like

select i.itemlookupcode, sum(quantity) from transactionentry te inner join [transaction] t on te.transactionnumber = t.transactionnumber inner join item i on te.itemid = i.id where t.time > (week to date) group by i.itemlookupcode

- - week to date being the first day of the current week - sunday

I appreciate any help I can get on this - thanks