tats true, but i am wondering how come select date_trunc(current_date) worked without any error? the actuall syntax is

select date_trunc(units, col).

There is no function named "date_trunc" that takes a single argument.  The date_trunc function takes two arguments:

select date_trunc('day', current_timestamp)

If you want to create a custom function that does the same thing as the Oracle trunc(sysdate) function,  here is an example of creating one using nzLua (works on any 5.x or 6.x system).  nzLua can be obtained by emailing netezza-nzlua@wwpdl.vnet.ibm.com or by joining the netezza developer network at www.netezza.com/ndn and searching for nzlua.  Note that when joining NDN you should use a real work email address, not a personal address such as gmail,  yahoo, etc.  Otherwise account creation may take a long time (or may never happen at all).

[nz@netezza ~]$ cat trunc.nzl

function evaluate(ts)
    return date_trunc('day',ts)
end

function getName()
    return "trunc"
end

function getType()
    return "udf"
end

function getArgs()
    args = {}
    args[1] = { "ts", timestamp }
    return args
end

function getResult()
    return timestamp
end

[nz@netezza ~]$ nzl trunc.nzl

Compiling: trunc.nzl
####################################################################
UdxName     =  trunc
UdxType     =  UDF
Arguments   =  TIMESTAMP
Result      =  TIMESTAMP
Dependencies = LIBNZLUA
NZUDXCOMPILE OPTIONS: (--unfenced --mem 10m )
CREATE FUNCTION
Created udf
Done

[nz@netezza ~]$ nzsql -c "select trunc(current_timestamp)"

        TRUNC
---------------------
2010-08-29 00:00:00
(1 row)