Today I try to use Wallch as alternative for Variety, but it's hard to install and config. So I switch back to Variety.
In Preferences of Variety -> Effects -> Clock, you can add a desktop clock using Variety. So conky is no longer needed.
The default position of this clock is "SouthEast", you can modify it in ~/.config/variety/variety.conf. If you modify "gravity" paramater in "clock_filter" to "NorthEast", the clock will be placed in higher right corner. See full description of customization of this clock in Configuring the Clock.
On Ubuntu 14.04 Unity desktop, I choose "Dejavu Sans Mono 70" for clock font, and "汉仪楷体 24" or "" for date font.