MySQL, YearTD/MonthTD/WeekTD Query?

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I have a MySQL database which contains a varchar field that stores dates in the format YYYY-MM-DD.

I need to do a count of the following records

Year To Date
SELECT COUNT(*)
AS ytd
FROM termine
WHERE von=$user_id
AND datum= !!---FIRST 4 DIGITS = CURRENT YEAR!!

MONTH TO DATE
SELECT COUNT(*)
AS mtd
FROM termine
WHERE von=$user_id
AND datum= !!---CHARACTERS 6+7 = CURRENT MONTH!!

WEEK TO DATE
SELECT COUNT(*)
AS wtd
FROM termine
WHERE von=$user_id
AND datum= !!---NO IDEA!!

Any help would be greatly appreciated, I can define $current_year, $current_month vars if they are needed, but I think this can all be done with MySQL?

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I have the YTD and MTD figured out...But I still need help with the Week To Date...

SELECT COUNT(*) AS ytd FROM termine WHERE von=$rows[user_id] AND LEFT(datum,4)=2003");

SELECT COUNT(*) AS mtd FROM termine WHERE von=$rows[user_id] AND LEFT(RIGHT(datum,5),2) = MOD(MONTH(NOW()), 12)");

[barand]

This function will give the timestamp for the previous sunday

function lastSunday() {
    $da = getdate(time());
    $ndays = $da["wday"];
    return time() - ($ndays * 24 * 60 * 60);
}

You can the query for dates > date('Y-m-d', lastSunday() )