PHP sql Subtract two dateTime fields to a millisec

[neelsfer]

I have two fields in a sql query for a html table that needs to be subtracted. The datetime format looks like this "19/08/2017 23:32:08.580"
My sql with inner joins looks like this

$query = "SELECT tblentries.categories,tblracedetails.actual_start_time,tblentries.age,tblentries.firstname,tblentries.surname,tblentries.distance,tblentries.raceid,tblracetime.finishtime,tblracetime.raceno,tblentries.racedate,tblentries.racename,TIMEDIFF('tblracetime.finishtime','tblracedetails.actual_start_time') AS racetimes
FROM tblentries
INNER JOIN tblracetime
ON tblentries.raceid = tblracetime.racesid AND tblentries.raceno = tblracetime.raceno
INNER JOIN tblracedetails
ON tblracedetails.id = tblentries.racesid AND tblracedetails.distance = tblentries.distance
";

The data is stored in a varchar fields called "actual_start_t ime" and "finishtime ". The final subtracted format should be h:m:s:ms and the subtracted value must be called "racetimes" .
The sql query above displayed, works 100% except for an empty 00:00:00 racetimes column.
Any suggestions will be appreciated? Thx