In ICC, we can report clock skew using following 3 commands: report_clock, report_clock_timing, report_clock_tree. Here is the differenc using those three command to report clock skew.
1. report_clock -skew
Reports the skew constraints for the clock set by the set_clock_uncertainty command.
2. report_clock_timing -type skew
Reports the actual local clock skew, that is, the skew between two flipflops of that clock, which has a timing path between the flipflops. This report is more useful to determine the real effect of the clock skew.
3. report_clock_tree -summary
Reports the actual global skew, which is the worst skew across the entire clock network. The report_clock_tree command is not OCV-aware and does not recognize set_timing_derate, whereas report_clock_timing -type skew is aware of both OCV and timing derating. Also, the report_clock_timing command does not consider float pin delays, whereas the report_clock_tree command considers float pin delays when reporting maximum global skew.