Reg Slack calculation

start clock and end clock sounds more like a simulator than static timing analysis, but I may not understand the question. Clocks are usually controlled by:

create_clock -period 10.0 -name clk_name [get_ports clk_name]

Internally they are constrained with derive_pll_clocks. I would recommend the following(I wrote it, so biased) for getting started:

http://www.alterawiki.com/wiki/timequest_user_guide