The 555 isn't going to be your best bet if you need anything close to precise
timing because the timing will drift based on both tolerance of the RC
components and the temperature. Also it isn't easy to get a 50 percent
duty cycle with the 555.
If this isn't battery powered, one of the most reliable source of time is
the power cycle from the wall. A center tapped transformer, along with a
single current limiting resistor and a zener diode, is all that's required
to get near rock solid 60 Hz pulses that you can divide/count down to your
heart's content. The transformer also will provide power to your project
with a bridge rectifier, voltage regulator, and an additional couple of
caps.
I use the above setup to drive any precise/long term timing PIC projects I
put together. Works like a champ.
This howstuffworks.com page describes the exact setup I'm talking about:
http://home.howstuffworks.com/digital-clock4.htm
Of course I believe the rest of the circuit past the power supply/60 Hz
generator falls down because it's a perfect application for a microcontroller.
Using discrete TTL, while useful as a learning exercise, is impractical for
real implementations. However if you're going discrete, pay attention to the
divide by 6 and divide by 10 on the subsequent pages.
Howstuffworks also talks about how to build a digital clock with a basic stamp.
They point out that the setup will cost $100. Insane. Fortunately you can
skip the stamp and program to an inexpensive bare microcontroller. Presuming
you're only wanting the same 1 Hz output as the 555, a 16F628 can be had
for less than $3, or on sample from Microchip.
BAJ