Sorry, I've made a couple of miscalculations... It's too early in the morning. I realized it after I tried making an example code and ended up with 1,000,000 seconds...
Firstly, calculate the hour with: $calc($time(H) * 60 * 60)
Then calculate the minutes with: $calc($time(nn) * 60)
Then finally add it all together: $calc(%hrs + %mins + $time(ss))
This should return the right time now.
