I am replacing an old wall timer with a GE Smart Digital Timer. The old timer had two black wires connected to two black wires from the wall. The new timer however has red, black, white and green wires. I know the green is the ground, but how do I connect red, white and black to just two black wires?

It sounds like you are replacing 3-way switch.
Low voltage light? Are these lights connected to 120Volt circuit?

I think you want the GE 15312 timer, instead of GE 15313.
http://waterheatertimer.org/How-to-wire-GE-15312-timer.html

GE 15313 is for single-pole incandescent light bulbs only.

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You are install GE 15313 timer
Open following link to identify timer
http://waterheatertimer.org/GE-timers-and-manuals.html#15312

15313 timer does not require Neutral wire.
The timer you are replacing has neutral wire (White wire)
So replace timer wires 1:1 except do not connect timer to white neutrals.

Keep in mind the 15313 is for incandescent lights only and is not for CFL or LED lights.
If you want timer for CFL, then install one of the other timers shown on page.

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I test these timers.
GE 15071 timer has internal clock that runs on electricity. That's why timer has extra wire.
Intermatic, GE, Tork, Leviton, Sylvania, Woods, Utilitec etc in-wall timers each wire the same way.
The only tiny difference is GE doesn't have green ground wire.
http://waterheatertimer.org/Program-wire-Utilitec-0192773.html

Here's what to do:
1) Old switch has two screws (excluding green ground screw)
One screw on old switch has 2 black wires >> these are hot wires > timer black wire connects to these hot wires. This will power the timer.
2) The other switch wire connects to timer red wire. This will send timer signal to lights.

3) Timer white wire is for the timer clock.
Timer white wire connects to other white wires that are twisted together and covered with wire nut >>> these are neutral wires, and now timer clock will operate
4) For EZ connection to other white wires, strip back another inch on timer white wire so stranded wire can wrap like a candy cane around solid copper wires. Then twist wire nut on very tight.

5) When wiring is complete, push door on timer for manual override. Lights should turn ON-OFF. If lights do not turn on, reverse timer red and timer black wires. Make sure outdoor bulbs are working.

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It sounds like wiring is wrong.

If you have GE Tork Intermatic UPM Woods Brinks Hagar Pass-n-Seymour Stanley Westek in-wall timer, with red, black, white and green wires, then you probably put red and white wires together.
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See instructions along with image at following link:
http://waterheatertimer.org/Program-wire-Utilitec-0192773.html

Heres how to wire timer
1) Timer-green-wire connects to bare ground wire.
2) Switch has 2 wires. Take switch wires and connect them to timer-black-wire and timer-red-wire. Choose either one for now.
3) In back of electric box are white wires twisted together and covered with wire nut. Connect timer-white to these wires.
4) If electric box does not have white wires described above, then connect timer-white to bare ground, along with timer-green wire.
5) Turn on electricity, and push ON-OFF button > with GE timer, door is push-button switch. If lights come on, timer is wired correctly. If lights do not come on, then reverse timer-red and timer-black wires.

GE -15265 manual
http://waterheatertimer.org/pdf/GE-15265-manual.pdf
Ah, the usual how to connect 4 wires to the old 'was-3-wire and now-2-wire' problem.

I tested and wired these timers.
Aube timer has internal rechargeable battery, and therefore does not need another wire to run internal clock.
GE timer has extra wire to run internal electronic clock, just like alarm clock is plugged in.

Heres what to do in simple format
1) Timer-green-wire connects to bare ground wire.
2) Old timer has 2 wires. Take both wires and connect them to timer-black-wire and timer-red-wire. Choose either one for now.
3) In back of electric box are white wires twisted together and covered with wire nut. Connect timer-white to these wires. This action will complete circuit to internal clock.
4) If electric box does not have white wires described above, then connect timer-white to bare ground. This works because bare ground and white neutral both connect to neutral busbar inside breaker box.
5) Turn on electricity, push in timer door for manual override. If lights come on, timer is wired correctly. If lights do not come on, then reverse timer-red and timer-black wires.
If light does not come on, double check that outdoor bulb is good, and that breaker not tripped.
Timer can also be reset in event of short. Reversing red and black wires will not short timer.
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http://waterheatertimer.org/See-inside-main-breaker-box.html

I've tested these timers.
Here's what to do:
1) Connect green to bare copper
2) Switch has 2 wires. For now, connect these two wires to timer-black and timer-red. Choose either one.
3) In back of wall box are white neutral wires twisted together and covered with wire nut. Timer-white wire connects to these white neutral wires. This action gives power to time clock.
4) If wall box does not have white neutrals described above, then connect timer-white to bare ground wire. Bare ground connects to neutral busbar in breaker box.
5) Turn on power and click manual override button or switch. If lights turn on, timer is wired correctly. If lights do not turn on, then reverse timer-red and timer-black wires.

Manual for GE 15265
http://waterheatertimer.org/pdf/GE-15265-manual.pdf
15A timer works fine on two bulbs, up to 1440 total watts.

If old switch has 2 screws plus green ground screw, then timer is right.
If old switch has 3 screws plus green ground screw, 15265 doesn't replace 3-way switch.

Timer-green wire connects to bare ground wire

Old switch has 2 screws with wires on each.
One wire is Hot-from breaker and this connects to timer-black wire.
One wire Goes-to-lights and this connects to timer-red wire.
If you get these two wires reversed, timer will not work, so reverse them.
It will not blow circuit if the timer-red and timer-black wires are reversed.

Timer-White wire is the confusing wire for everybody:
Timer-white wire runs clock motor located inside timer.
In back of electric box are neutral white wires that are twisted together and covered with wire nut.
Connect timer-white to these neutral wires.
If box does not have neutral white wires, connect timer-white to bare ground wire.

GE 15086 is a 7-day programmable timer. It has more wires than spring-wound timer because clock motor runs on 120V circuit, just like ordinary electric clock. The clock on spring wound timer operated by a spring.



If I understand correctly, the old timer has 2 wires. One of these wires is the Hot wire ... and that wire will connect to GE timer black wire.
The other wire from old timer goes to Load ... and that wire will go to GE timer red wire.

Instructions on a lot of these timers say, it timer doesn't work, then reverse the two wires described above ... this is because timer only works when Timer black connects to Hot.

You can also turn on power and test each wire that came off old switch to bare ground wire. When tester lights up, that is Hot wire.

GE timer Green wire connects to bare ground wire.

Moving on to GE timer white wire. This wire is necessary for the clock to run. This wire connects to Neutral wire which is usually white. You can test for Neutral by testing Hot wire to each white wire in box ... when tester comes on, that is Neutral.

If you do not have a white Neutral wire, then connect GE timer white wire to bare ground wire. This is not code, but it will work fine until you hire electrician to run Neutral into box.

EJ341 has 2 wires

The timer is wired incorrectly.

ON the GE timer and Utilitec timer: The red and black wires are reversed.

On the Intermatic ST01C