555 Timer

Sorry if this appears twice - I had replied but it dissapeared

You need a couple of other chips as well as the timer, a BCD counter (7490) and an LED driver IC (7447)
There should be lots of 555 timer examples on google or in the library.

 

How long do you want to time and how accurately.
A 555 is great for timing a few seconds to minutes but if you need to accurately time hours or days the temperature tolerances on something like a 555 will be a problem. It would be better to use a dedicated timing chip and a micro.

ps Here is a good example page for the 555
http://www.uoguelph.ca/~antoon/gadgets/555/555.html [Broken]

 

I would find an off the shelf timing chip, either a frequency counter or time-digital-converter.
The long term stability is the more difficult thing to acheive, it is very easy to time 0.1ms intervals upto 1sec but if you needed to time something lasting hours you have to consider the temperature stability of the components. An off the shelf timing chip will have temperature corrections built-in, a 555 timer would depend on the behaviour of all the external components.

Look for a book called "The art of electronics" in your library it is a very good introduction to this sort of electronics.

 

I think crystal oscillators are typically used when you need something more accurate than a 555 timer.

 

You can get crystal oscillators of different frequencies. Choose the one which you want and it will be the external clock to your circuit.