I have not used mechanical timers in a while since I found out about these small 7-Day Astronomic timers, 3 year battery back up, adjust to changes of seasons as they calculate sunrise and sun set automatically so no photo cells ever needed, they will handle a 20 amp resistance load, but for most applications I use a small Square D contactor that can be purchased less then $30.00 these timers can be obtained online for around $40.00 and have been very reliable, what I like about them is that they are a set and forget timer, no being called out to reset the time after a power failure, the program is very easy to set if you follow the instructions, and very easy to modify, but you must set it up in the steps outlined in the instructions.
Here is where I get them from:
http://www.smarthome.com/25089AL/Intermatic-EI600LAC-7-Day-Electronic-In-Wall-Timer-Almond/p.aspx
When I install them outdoors I just use a WP box and a in-use cover for a GFCI receptacle, and install the small contactor in a small 4x4x4 PVC JB box.