Which hook to use when a date is going to be saved into the database I have a situation where I would like to know when a date (for an event) is going to be saved into the database so that I can modify the corresponding date in another module to match it. That is it in a nutshell. I need to do this because of the way this other module works. What is the best hook to use to accomplish this? Is it hook_node_update?

Thanks.
If you are only using nodes then yes, implementing hook_node_update is a good approach. You also will need to implement hook_node_insert if you need to do logic when the nodes are first created.