The greatly appreciated expertise of this forum guided me to the following solution:

SS7C

The switch works like a charm for my outdoor house lights. I wanted to add a second timer switch (for outbuilding) to a separate circuit (has 3-way switch, 1 in outbuilding and 1 in house), however found out in the manufacturer’s fine print that it will not work for wiring runs where the remote toggle switch is more than 30 feet from the timer switch, or for wiring runs that are underground (I’m DQ’d on both conditions!). Out of curiosity, what's the explanation for why the timer switch will not work under these conditions?

Thanks in advance.

Quoted: The greatly appreciated expertise of this forum guided me to the following solution: SS7C The switch works like a charm for my outdoor house lights. I wanted to add a second timer switch (for outbuilding) to a separate circuit (has 3-way switch, 1 in outbuilding and 1 in house), however found out in the manufacturer’s fine print that it will not work for wiring runs where the remote toggle switch is more than 30 feet from the timer switch, or for wiring runs that are underground (I’m DQ’d on both conditions!). Out of curiosity, what's the explanation for why the timer switch will not work under these conditions? Thanks in advance.

The requirements are set that way because the timers "talk" to each other via a control wire, which is a low voltage signal line and dependent on the master's ability to "push" the information signals through the high voltage line run parrallel in the series circuit.  The longer the wire run, the more resistance is in the circuit.  This resistance interferes--and even negates--the controller's ability to "talk" to one another clearly.

Just like if you and I were standing too far apart, we might be able to hear each others' voices, but we wouldn't be able to make out the words because the syllables and inflections are lost over the distance.

Mechanical timers do not have this setback because their transmission requires no signal - just a manual toggle of the high-voltage lines between the open and closed positions.  Using the above analogy, you and I using bullhorns would be able to hear each other at a distance our voices would barely carry, making our intent inaudible.  

Put another way, remember the old two-cups-and-a-string phone game we did as kids?  Remember it only worked in a very limited distance?  That's because the friction of the sound wave transmission diminished over the distance to surface ratio:  resistance.  Same with speaker wire.  But this is called frequency.  Same with dropping a rock in water and the resistance makes the ripples smaller and smaller the further out they go.

In this case, though, the actual problem is called "harmonics."  Harmonics are the sine wave interference with direct, low-voltage transmissions.  As electricians, we battle harmonics in installations that are elctronic-sensitive, as the magnetic pull of AC high-voltage lines will interrupt--and even cancel--low-voltage applications.  This is known as "harmonic inbalance," which essentially means the frequency of the high-voltage lines makes too much magnetic impedence for low-voltage lines to transmit freely.  The longer the run, the more the resistance; the more the resistance, the weaker the signal; the weaker the signal, the greater the voltage needed to keep the ampers necessary; and the more the amperes, the greater the frequency Ohms interference.  

To overcome this, you have three options:

Switch to manual timers, or;

Install a 4-way system with slave timers no further than 30 feet apart, even if you don't use them.  This could get expensive if the distance is considerable, of course, and you need a place to mount them;

Or, go to a higher quality timer that signals the slave (remote) timers at a higher signal strength, thus allowing you to move the units further apart.  

The underground portion makes no sense to me.  But I'm HARDLY an engineer, so maybe someone can correct any errors I made and explain the underground stipulation.  

Thanks for the thorough explanation and ideas.