PHP - Comparing date, checking if within last hour

if(time() - strtotime($unit['last_accessed']) < 3601){

or

if(time() - strtotime($unit['last_accessed']) > 3599){

time() and strtotime() both use the same time base in seconds

if $unit['last_accessed'] is already an integer time variable then do not use strtotime().

First of all, you're using date function in a wrong way. From PHP's manual:

date ( string $format [, int $timestamp = time() ] )

You must provide as first argument the $format string and the $timestamp as the second. You can check if the time is whitin the last hour without transform the Unix Timestamp to a another timestamp string.

$last_hour = time() - 60*60; //last hour timestamp
if($unit['last_accessed'] >= $last_hour){
       $connect = "<i class='fa fa-circle 2x' style='color:#2ECC71;'></i>";
}else{
       $connect = '';
}

As you can see, i didnt made any transformation to the timestamp, as i'm not using the timestring in anywhere. You should learn a little more about operations with unix timestamp's or about php time functions.

References: http://php.net/manual/en/function.date.php