Programming arduino to store even time?

I have a project I want to try out which is basically a laser tripwire. I want the arduino to log the time a laser is tripped. Wither connected to the computer or not it doesn't matter to me. I'm thinking it would be easier to log the dates on the computer so lets try that.

So anyways how would I go about doing this? I got the time library and managed to use an example from it to make a speaker produce a tone at a certain time but the speaker is dependent on the time. I want the logged time to be dependent on the laser tripwire which I so far don't know how to do.

Now I guess the basic process is: 1. Start the time and have it count 2.When laser is tripped have the arduino look what time it is and save it (on computer or eeprom if possible) 3. keep on counting ready for the next time it will be tripped.

So yeah can someone point me in the right direction as to how I can incorporate a logging function to save the time and also how to actually make it dependent on the laser being tripped?

void loop ()
  while (! laserTripped());
  logTime ();

maybe use - - my favo logshield, has SDcard and an RTC.