PHP time confusion with if statements

I'm sure this is a very obvious question however I am really bad at working with timestamps and comparing them to in if statements.

What I am trying to do is define if a staff member has been on the team for 0-5 months 6-11 months 12-17 months and 18+ months

And that is just meant to change the class of the table so

0-5 months is RED

6-11 months is ORANGE

12-17 months is BLUE

18+ months is GREEN

Here's my code so far, I'm sure it's completely wrong. The $dataResult['hire_date'] is the timestamp when they were hired.

$milestone6 = strtotime("6 months ago");
$milestone12 = strtotime("12 months ago");
$milestone18 = strtotime("18 months ago");

if($dataResult['hire_date'] < $milestone6) {
    //red?
} elseif($dataResult['hire_date'] < $milestone12) {
    //orange?
} elseif($dataResult['hire_date'] < $milestone18) {
    //blue?
} else {
   //green?
}

Again thank you. It does not matter what order the if statements are in. Will mark as answered when I can.