How does Qualtrics set the survey duration for unsubmitted/unfinished surveys?

Hi there, Duration is the difference in seconds between StartDate and EndDate, where StartDate is when the survey was created and EndDate for incomplete responses is the last time the survey was interacted with.  
If the survey's Incomplete Survey Responses setting is set to "Last time they edited a response" and not "Survey Start Time", it is possible for response Duration to be longer than the survey's Incomplete Survey Response Recording Timeframe if the respondent returns to the survey session and edits their response, restarting the timer that would otherwise record their response and extending the survey's Duration.
If you have a lot of responses with a duration above 172800, it might be worth having Qualtrics support take a look at it.

Based on Qualtrics' definition, I believe StartDate is when the user opens the survey page for the first time.
"The number of seconds it took the respondent to complete the survey. This is the entire duration of the response; if a respondent stops in the middle of the survey, closes the browser, and comes back another day, that time is counted."
If StartDate was the survey creation date I would expect many thousands of records with the same timestamp.

Sorry, I meant the "survey session" being created, which is when the respondent first clicked into the survey.