It seems that MySQL cannot cast a date as float (as per the other examples in stackoverflow). Perhaps the other examples refer to another database. So by converting dates to unix_timestamps I am able to get an answer...with the final calculation in PHP. If this is WRONG...please post and I will remove answer...
SELECT
COUNT(*) AS N,
SUM(UNIX_TIMESTAMP(LOGENTRY.DATE)) AS Sum_X,
SUM(UNIX_TIMESTAMP(LOGENTRY.DATE) * UNIX_TIMESTAMP(LOGENTRY.DATE)) AS Sum_X2,
SUM(LOGENTRY.COST) AS Sum_Y,
SUM(LOGENTRY.COST*LOGENTRY.COST) AS Sum_Y2,
SUM(UNIX_TIMESTAMP(LOGENTRY.DATE) * LOGENTRY.COST) AS Sum_XY
FROM LOGENTRY