The Days on Zillow field is very 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.
For more information, see the link below:
How is the days on Zillow counter determined?