This simple calculator allows you to calculate time intervals. Now it supports only positive numbers. most buttons have keyboard shortcuts. It can be particularly useful to pack audio tracks on magnetic tapes or disks. You can enter time in seconds, minutes, hours and days. When unit is omitted, seconds are assumed. For example to enter one hour and 15 minutes interval, you can type something like: "1h15m". To add 40 seconds to it, just push '+', then, "40" and then "=".

You can also take a look at the source.