Most likely the value in the database is empty and not 00:00:00.
try using this for the if statement instead:
if ("".$row_calendar['event_time'] ."" != "") { // WebAssist Show If
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.