The scheduled dashboard report emails will pull the report based on the set number of days in the Date filter based on the date the email is being sent. 

For example, if you set the Action Date filter in your dashboard to 1/1/2017-5/22/2017.  This is 142 days. 
If you schedule the dashboard to send an email with the dashboard report on a weekly basis, NXT will update the Action date filter to 1/7/2017-5/29/2017, which reflects 142 days. 
The following week, the dashboard report would use the date range 1/14/2017-6/5/2017, which reflects 142 days.

This is working as designed.  The Analyze panels are dynamic and the email feature is matching the date range as time moves forward. If a set date range is desired without it being modified, then they will need to login to NXT directly to view the Analyze panel or if the Panel is being sent to others, download the panel as a PDF and email it directly.  

Our Product Developers are working to improve the features and functionality of emailing the Analyze panels.  You can add this as a suggestion on our Community page at
https://community.blackbaud.com/products/raisersedgenxt

Our developers check here first when considering program enhancements and features to see what our users are most interested in seeing implemented.  You can review other ideas and vote for them and of course if your idea is already listed, be sure to add your vote.