As Jim said the problem is with DATE(), you must convert the date to datetime, If you didn't care about time portion it is easier, but if you want the exact datetime for furter calculations uyou must add zeroes there:

mystr = "Insert into mastcust (masterref,amenddate) values('0000000001',?m.mydate)"
sqlprepare(handle,mystr) && Ok so far

***m.mydate = date()
** If you didn't care about time portion, the DATE portion of this field is used only for info
m.mydate = DATETIME()
** but if the DATE portion of this field is used for some calculations:
m.mydate = DATETIME(YEAR(DATE(), MONTH(DATE(), DAY(DATE(),0,0,0)
retval = sqlexecute(handle)
IF retval < 0
   AERROR(arError)
   DISPLAY MEMO LIKE arError
ENDIF