I managed to solve the problem as follows
 

-- current day

WHERE (Timestamp >= {fn CONVERT(CURRENT_DATE,SQL_TIMESTAMP)}
AND Timestamp < {fn CONVERT(DATEADD(dd,1,CURRENT_DATE),SQL_TIMESTAMP)} )

-- previous day

WHERE 
(Timestamp >= {fn CONVERT(DATEADD(dd,-1,CURRENT_DATE),SQL_TIMESTAMP)}
AND Timestamp <  {fn CONVERT(CURRENT_DATE,SQL_TIMESTAMP)});