The Days on Zillow field is similar to the CDOM field on the MLS. The number of days counter is meant to show consumers how long the property has been listed on Zillow. A listing must be "off market" on Zillow for 31 consecutive days or have been relisted with another listing agent in order for the Days on Zillow field to reset to 0 for a new listing.
Additionally, now that Zillow is a brokerage/IDX site and not a consumer portal, they receive and display Coming Soon listings. Any time a listing is in the Coming Soon status on the MLS will be included in the Days on Zillow figure, since it becomes visible on Zillow shortly after it is saved as Coming Soon on the MLS.
For more information regarding Days on Zillow, see the link below:
How is the days on Zillow counter determined?