how to find out all the activities happend in a branch in the last month?

Do not forget the time directive of selection rule.

element /myPath/... /main/{!created_since(16-Sep-2009)}
element /myPath/... /main/LATEST

Can enable you to see versions from a month ago (see time and query)

See also "time-based rule" answer.

From there you can branch and update your code from this older stable point.

The "lshistory" command can be used to see the list of events that were performed (create version, create branch, etc).

ct lshistory -since 16-Sep-2009 -branch your_branch -recurse /path/to/vob

This would show all events that happened in /path/to/vob and all files and directories under that happened on your_branch since September 16th, 2009.

Adding "-short" will show the output as a list of elements and their version extended pathname, without the comments and other information.