If the date is being generated by a report rule and you are exporting it to excel, the following instructions will allow the date and time to be shown as only a date.

You may be able to tailor these for your specific purpose of hiding just the day:

1. Click gear icon on property row of report rule.  Type “Date” and then click “best bets” instead of “all matches”
2. Choose the only choice offered that says “Date Time    pxDateTime”.  
3. Click magnifying glass and pick type “Date”, format “Custom” and type in “MM/dd/yyyy” (yes, you need uppercase M’s and lowercase d’s and y’s)

/Eric