Why do outdoor Christmas lights turn off after 2 hours?

Westinghouse outdoor timer instruction manual
http://www.manualslib.com/manual/190259/Westinghouse-Outdoor-Timer.html

Westinghouse Outdoor Timer User Manual

The Intermatic outdoor timer 1076311 is Intermatic model HB65R.
It has two buttons, TIME and MODE and a digital display.
Instructions:
1. Plug in the timer, which self-tests. If error, EE flashes.
If normal, 88 flashes for 3 seconds, then display shows C and you can proceed to set the timer.
2. Push the TIME button to set C (current time) AM hours 1-12 and PM hours 1. to 12. (Example: for 7 AM push the TIME button until the display shows "7" for 7 PM push the TIME button until the display shows "7."
3. Now push and hold down the MODE button until the display shows P for the "ON time."
4. Push the TIME button to set the hour to turn ON, from AM 1 to 12 or PM 1. to 12.(Example: if you want the power on at 7 PM repeatedly push the TIME button till the display reads 7.)
5. Push and hold down the MODE button until the display reads E for "END time" (time you want the power OFF)
6. Push the TIME button repeatedly to set the end time hour from AM 1 to 12 or PM 1. to 12. The time off will display.
7. Press the MODE button to toggle between ON and the current time.

If you unplug the timer you will have to reset the C (current time), P (ON time) and E (END or OFF time).

Here is how it works. The push button repeats itself in following sequence (must plug in).
F 1 2 3 4 5 6 7 8 9 A d 1. 2. 3. 4. 5. 6. 7. 8. 9.

F: timer is off.
1 2 3 4 5 6 7 8 9: from dusk to dawn count down number of hours repeated every day. That's most useful to control outdoor lightings.
A: from dusk to dawn count down number of hours which is randomly selected by this timer. Which is usually for indoor use to turn lights on and off for number of hours. The randomly selected hours is a security feature so that lights on (on from dust, but off randomly) time shows no pattern to outsiders.
d: just on from dusk and off at dawn. It is also used to control outside lights but just a little wasteful in my opinion.
1. 2. 3. 4. 5. 6. 7. 8. 9.: meaning count down number of hours starts from the time when the button is pushed so that you can have a countdown timer at any time.

You posted under Woods 59204 outdoor timer.
If this is not your timer, add a comment.
http://waterheatertimer.org/Woods-timers-and-manuals.html

manual for Woods 59204 timer
http://waterheatertimer.org/pdf/Woods-59204.pdf

Instructions:
Rotate dial until current time lines up with arrow.
48 segments (or pins) located around edge of dial > each segment is 1/2 hour
Set ON-off times by pushing pins in or out
Pins pushed out = timer ON
Pins pushed inward = timer OFF
Plug in timer.

Maximum 1250 Watts.
50 mini-bulb strand of Christmas lights is 25 Watts.
Timer will operate 50 strands of 50 mini-bulbs.
Long-winded guide for Christmas light wattage:
http://www.christmaslightsetc.com/pages/how-much-power.htm

Here is how it works. The push button repeats itself in following sequence (must plug in). F 1 2 3 4 5 6 7 8 9 A d 1. 2. 3. 4. 5. 6. 7. 8. 9.
F: timer is off. 1 2 3 4 5 6 7 8 9: from dusk to dawn count down number of hours repeated every day. That's most useful to control outdoor lightings. A: from dusk to dawn count down number of hours which is randomly selected by this timer. Which is usually for indoor use to turn lights on and off for number of hours. The randomly selected hours is a security feature so that lights on (I believe the on time is within the range of 1 hour from dusk set by the timer) and off (it is randomly set, you can see the display blinking showing the selected countdown hours) shows no pattern to outsiders. d: just on from dusk and off at dawn. It is also used to control outside lights but just a little wasteful in my opinion. 1. 2. 3. 4. 5. 6. 7. 8. 9.: meaning count down number of hours starts from the time when the button is pushed so that you can have a countdown timer at any time.

I couldn't find a manual for your Timex, but here are the setting instructions:

Clear the timer by pushing hour and minute at same time. To set current time, hold time button down and push hour. Then hold time button down and push minute.
To set the time for light to turn on, hold the ON button down and push hour until it gets to the hour you want it on (same for Minute). To set time for light to go off, hold the OFF button down while you set the hour and minute.

manual for HB51RC
http://www.intermatic.com/%7E/media/files/intermatic/products/instructions/timers/hb61rc%20-%20english.ashx
See timer at Amazon
http://www.amazon.com/Intermatic-HB61RC-Digital-Outdoor-Control/dp/B00004W4X5

Plug timer into outlet
Plug lights into timer
Timer turns on automatically at dusk unless you set override feature shown below

Timing cycle repeats each 24 hours
Use selector button to set timer
Timer must be reset after power outage or after unplugging timer.
Push selector button and choose one of following

F > Lights are OFF
O > Constant ON
1 through 9 > timer turns ON at dusk and turns OFF 1 to 9 hours later
d > timer turns ON at dusk and OFF at Dawn
1. through 9. > timer turns ON immediately and then turns OFF 1 to 9 hours later
Maximum wattage 1000
50 mini-bulb-string of Christmas lights is 25 watts

From these instructions, you should push button until 6 displays. Timer will turn ON at dusk and turn OFF 6 hours later
If timer.
If you want timer to turn ON immediately and turn OFF 6 hours later, push button until 6. shows in display.

If timer does not display or does not operate correctly, then timer is suspect. Return timer for another one.

manual for HB51RC
http://www.intermatic.com/%7E/media/files/intermatic/products/instructions/timers/hb61rc%20-%20english.ashx
See timer at Amazon
http://www.amazon.com/Intermatic-HB61RC-Digital-Outdoor-Control/dp/B00004W4X5

Plug timer into outlet
Plug lights into timer
Timer turns on automatically at dusk + override lets timer run during daylight hours
Timing cycle repeats each 24 hours
Use selector button to set timer
Timer must be reset after power outage or after unplugging timer.
Push selector button and choose one of following

F > Lights are OFF
O > Constant ON
1 through 9 > timer turns ON at dusk and turns OFF 1 to 9 hours later
d > timer turns ON at dusk and OFF at Dawn
1. through 9. > timer turns ON immediately and then turns OFF 1 to 9 hours later
Maximum wattage 1000
50 mini-bulb-string of Christmas lights is 25 watts

Here is the instruction manual for your model:

HB65R Timer

Hope this helps!

Also consult this website for sunrise, sunset information for your area:

Sunrise sunset calendar

Use the top button to adjust the current time & then push the bottom button once to save. Next is the time you want the lights to turn on & push bottom button once to save. The last setting is the time you want the lights to turn off. Don't forget to use the . (period) indicator for the pm setting. (I had the same problem and had to search the net to find someone else who had posted the instructions.)