use this
Query = "UPDATE tabDatafiveMinutely SET PAR0050=308.3925
WHERE TimeValue = CONVERT(DATETIME, '2012-05-16', 102)
for other date format
Date -100- MMM DD YYYY' -- Feb 5 2010
Date -101- MM/DDYYYY'
Date -102- YYYY.MM.DD'
Date -103- DD/MM/YYYY'
Date -104- DD.MM.YYYY'
Date -105- DD-MM-YYYY'
Date -106- DD MMM YYYY' --ex: 03 Jan 2007
Date -107- MMM DD,YYYY' --ex: Jan 03, 2007
Date -108- MMM DD YYYY' -- Feb 5 2010
Date -110- MM-DD-YYYY' --02-05-2010
Date -111- YYYY/MM/DD'
Date -112- YYYYMMDD' -- 20100205
Date -113- DD MMM YYYY' -- 05 Feb 2010
to convert date to text use like this
CONVERT(VARCHAR(12),getdate(),113)