TL:DR: If you modify a schedule scan, it mucks up your scan schedule if you have the scan repeating at any type of specific frequency. I was told by Qualys support today that the behavior I am seeing (where it used the date I modify the scan as the new start date vs my original start date) is actually current design behavior and they have no plans to change it....
This is poor design (IMO) and I would think anyone that uses this tool on a daily basis will tell you the same thing... If I plan a scan frequency around a specific start date, I don't want that to change if I make a minor update to the overall scan schedule. I have put in a feature request to force the "customer start date" to not be altered when editing scheduled scans: 624957. Feel free to ask you TAM to be included.
Backstory: We have a few scheduled scans that run on a quarterly schedule. I purposefully have them run in the middle of the quarters (so Feb, May, Aug, Nov). They were configured to run the X <insert weekeday> of every 3rd month. I made a new option profile I wanted to start using for scanning, so in Jan, I updated the scheduled scans... All of a sudden, my "next scan" says it is going to be in March. Playing around with scheduling and editing scans, it seems like once you edit a scheduled scan, Qualys uses the "last edit" date as the new start date/or reference date for the scan frequency.
As such, be vigilant when modifying scheduled scans as this has potential for kicking off scans when you are not expecting them.
Not sure how I never caught this before. What really disappoints me is how Qualys doesn't see this as a design issue. I feel like anyone that uses this tool on a day to day basis would see this as a huge issue. Support even told me I am not the first customer to bring this concern up.