What is the difference between contact date and last modified fields when you look at a specific delivery? If I want to know if my workflow created a delivery for that day should I be looking at contact date or last modified?
Contact Date : the date when the server starts the send of the delivery (can be scheduled in advance on delivery parameters)
Last Modified : as a lot of Campaign objects, is the date when the elements has been modified (i.e : the line on the table has been modified, with at least 1 column change). This date change during several time after the last message : the recipients activity lasts weeks (months ?) and new trackings appear (open, click etc.), so the delivery indicators are updated, impliyng a lastModified update.
If you want to know when your workflow created the delivery, you should look at the "@created" field, this is the real date of creation of a delivery (and a lot of other things in Campaign)
(beware : if you use the "continous delivery" mode, this won't be true)