How to record the real playing time?

Many games have such feature, in the save data, it shows how much time the player has played.

That means, in the save data, I have a variable called playTime. For example, when I opened this game for 5 minutes and closed, the playTime is 5 minutes; and next time when I play this game for 1 hour, the playTime would be 1 hour 5 minutes.

How do I achieve that?

The Time interface has ways to return the number of seconds the game or level has been running:

You will want to make sure Unity saves that data when the game exits, maybe detecting that with OnApplicationQuit:

There are lots of ways you could save the data between sessions. The PlayerPrefs interface is probably the simplest though:

In terms of converting that number of seconds value into something more human readable, in C# I would use DateTime:

But there are probably options in Unityscript/Boo that do the same thing.