Am I correct in assuming the campaign started on April 1st 1918?

If this is correct how do we put a date to each scenario?

For example if scenario 1 (Where are we) starts on April 1st, does mine begin on April 2nd? Or can I make a date up myself, if so how much of a gap between missions?

Just so I dont go and put a ate that is not reasonable for the campaign progression.