Can a presidential election be stalled or waived due to a declaration of war and has it ever happened?
Actually, neither of the answers below are accurate. Article 2, section 1, clause 4 of the Constitution states, "The Congress may determine the Time of chusing the Electors, and the Day on which they shall give their Votes; which Day shall be the same throughout the United States." (note: odd spellings and capitalisations in the original.) Thus, although the congress does have the authority to change election day, for any reason, not just a war, this has never happened except to the extent that they have permitted the states to use early voting. Given that we went through the Civil War with both sides holding regularly scheduled congressional and presidential elections, I think that if Congress ever wanted to make a signigicant change to election day they would have to have a darned good reason for it and enact it at least one full election cycle before the change was to take place in order to demonstrate that they were not up politically nefarious ends.
Actually, the answer below was only partly correct. Yes, no presidential elections have been stalled or delayed due to a declaration of war. But, the Constitution does not provide for a change in the elections for any reason.