It depends on what datatype you set for your db table.
DATETIME (datatype)
MYSQL
INSERT INTO t1 (dateposted) VALUES ( NOW() )
// This will insert date and time into the col. Do not use quote around the val
PHP
$dt = date('Y-m-d h:i:s');
INSERT INTO t1 (dateposted) VALUES ( '$dt' )
// This will insert date into the col using php var. Wrap with quote.
DATE (datatype)
MYSQL
INSERT INTO t1 (dateposted) VALUES ( NOW() )
// Yes, you use the same NOW() without the quotes.
// Because your datatype is set to DATE it will insert only the date
PHP
$dt = date('Y-m-d');
INSERT INTO t1 (dateposted) VALUES ( '$dt' )
// This will insert date into the col using php var.
TIME (datatype)
MYSQL
INSERT INTO t1 (dateposted) VALUES ( NOW() )
// Yes, you use the same NOW() as well.
// Because your datatype is set to TIME it will insert only the time
PHP
$dt = date('h:i:s');
INSERT INTO t1 (dateposted) VALUES ( '$dt' )
// This will insert time.