Error Putting a DateTime from php to mysql db? -

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i have time (h:i:s format), not date, call:

 $dateformat = 'y-m-d h:i:s';  $value4 = date($dateformat, strtotime($_post['start_time']));  $value5 = date($dateformat, strtotime($_post['finish_time'])); 

then call sql function:

$sql = "insert times (site_id, crew_leader_id, service_number, start_time, finish_time,                              deicer_quantity, salt_quantity) values ($value1, $value2, $value3, $value4, $value5, $value6, $value7)"; 

i following error:

error: insert times (site_id, crew_leader_id, service_number, start_time, finish_time, deicer_quantity, salt_quantity) values (3, 1, 1, 2016-11-06 15:00:00, 2016-11-06 15:15:00, 0, 0) have error in sql syntax; check manual corresponds mariadb server version right syntax use near '15:00:00, 2016-11-06 15:15:00, 0, 0)' @ line 3 

the datetime in correct mysql format, won't go db. ideas why? i'm running latest php , mysql. alternatively, if try convert datetime in mysql datetime string via unix_timestamp('datetimestring'), fills table 0000-00-00 00:00:00. heck doing wrong??

-thanks in advance!