I aim to create a digital clock using entirely 555 chips for the active circuits. Anywhere a semiconductor is needed, a 555 should be preferred, and care should be taken to minimize the number of non-555 semiconductors used.

Limited exceptions can be made where 555s cannot handle the required current (e.g. in display circuitry and power rectification). Nevertheless, the simplest component needed for this application should be used. Perhaps parallel 555s can be used for this.

For parts-count and board-area reasons, 556 parts (dual-555 timers in 1 package) are acceptable.

For power-dissipation reasons, I intend to use CMOS 555s, but I aim to make the design accept the use of bipolar 555s without any circuit changes (except maybe in the power supply).

In addition, the clock should keep good time. This means no RC delays should be used in the core timing functions, since they aren't precise or stable enough to rely upon. (Ironic, because typical uses of 555s need RC delays for monostable or astable operation.)

Exceptions: I might use a couple monostable 555s as a filter in the first stage of the clock oscillator, and maybe as switch debouncers. And (stretch goal) an astable 555 with an LDR for PWM dimming the display brightness to accommodate room lighting changes. None of these applications require precise, stable timing.