I have the ET420C that I want to run about 2 hours per day to chlorinate my spa. I cannot figure out anything from reading that manual. How do I set it up to run every day for about 2 hours?

ET420C manual is longer than space allowed for free answer.
Moreover, this manual leaves off key steps and is totally unclear, just as you discovered.
http://waterheatertimer.org/pdf/ET420C-Manual.pdf
On-line manual has page numbers, but pages are not in order, so look carefully at page numbers.
http://waterheatertimer.org/UPM-timers-and-manuals.html#ET420C

1) Let me work alongside you while we look go through manual.

2) This timer is simple electronic clock, like alarm clock.
Look at page 3 of manual about setting the clock. (Remember to look carefully at page numbers)
Clock needs to know what time it is at your house right now.
Manual shows 10 steps for setting current time.
You use arrow keys to set clock.

3) Next look at page 4 "program settings"
Use arrow keys to select program number
Program number 1 thru 20 appear on display

In my opinion, it appears that program1 is ON, and program2 is OFF
So program1 will be when you want chlorinator to turn ON
Program2 will be when you want clorinator to turn OFF
The manual is missing this key detail, so look at display to see if it shows program1 ON and program1 OFF ... if the ON and OFF do not display, then timer sets as described above.
Make sure you set same days for ON and OFF times >> if programs have conflicts, then timer will not operate as expected.

How to set programs:
Use arrow keys to select program1 and then press SET
Use arrow keys to select day, and then SET, and then arrow keys to select hour, and then SET, and then arrow keys to select minutes, and then SET.
Repeat steps for program2.

4) Mode appears on page 7 of manual
Again the manual is totally unclear how to set MODE or what buttons are pushed
I have the opinion that you do not set MODE unless you use Countdown or Random feature.
So see if timer operates without setting MODE

5) Add a comment if you get stuck, and I will review manual with you while you push the buttons.
You can add as many comments as needed, and I will respond as needed.

6) For heavy-duty outdoor timer with fewer functions, that is easier to program:
Up to 1/4 Hp, 1000Watts
http://waterheatertimer.org/Woods-timers-and-manuals.html#6-24
http://www.amazon.com/Woods-59403-Heavy-Duty-Outdoor-Digital/dp/B002KS7EOK
Or
http://www.amazon.com/Intermatic-HB800RCL-Seven-Outdoor-Digital/dp/B001KBZUNK

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