I'm a little puzzled by the following
echo "<p>" . time() . "</p>"; // current unix timestamp
echo "<p>" . time() - 60*60*1 . "</p>"; // 1 hours ago
echo "<p>" . date('H:i',time() - 60*60*1 ) . "</p>"; // 1 hours ago
returns
1351193453
-3600
20:30
Why is not time()-3600 evaluated when standing by itself?
It's an operator precedence issue. Subtraction is evaluated after concatenation. Wrap brackets around it, and it'll be fine:
echo "<p>" . (time() - 60*60*1) . "</p>";