It's easy to schedule out of office days in ZERO. Once you do, you will not receive a check-in email for these days. Here's how to do it:
1. Go to the check-in page. You can get there by clicking on the check-in email or by logging into ZERO and going to the Dashboard and then selecting "click here" in the check-in alert at the top.
2. On the check-in page, select the date after "When" (click on the light blue text).
3. Select the day (or days) you will be out of the office. Then click Save.
4. Select "Other" for "where are you working today?" and add a brief description (optional)
5. Then click on "Check In" at the bottom of the page.
You have now blocked off your out of office days, and you will not receive an email for these days.
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Other helpful tips:
You can always VIEW and CANCEL your future check-ins by going to the "My Check-Ins" page. Simply go to the Dashboard > select the "..." in the upper right hand corner of the dheck-in alert > then select "My Check-Ins" from the drop down list. 
On the "Active Check-Ins" tab, you can see you current and future check-ins. You can delete future check-ins by clicking on the red trash can in the table. You can also see your past check-ins and any check-ins you've created for colleagues from this page as well.