How can I timestamp my paper without violating double-blind review?
The most typical way to time-stamp a piece of work is to place it in a public repository: arXiv is a good example when it applies; many institutions also have a technical memo or technical report system in place.  Although this may technically violate certain interpretations of double-blind submissions, as noted in the comments blinding of authorship is pretty iffy in many cases.  Moreover, in practice this will only be violated if the reviewers actively go looking, particularly if you put it in an institutional repository rather than a global one like arXiv.  I would thus advise that if you are worried about date-stamping, just do it and let the double-blind nitpickers complain if they even notice.

One other consideration: certain publications (particularly certain journals) do have a policy against accepting material that is already available online.  While this is in my view an insane policy, if you find yourself dealing with such, one way to handle with it is to put up a shortened version of the work, i.e., extended abstract, as the date-stamped pre-print.