I am assuming you have two different reportlets for comparison. If yes, in your dashboard view click on 'Layout' and then click on the dates for the reportlet with rolling dates and select 'fixed' and update. Save the changes in your dashboard. That should do the job.
If not, please post a screenshot of your dashboard so that I can understand the layout.
I tried the same but on chosing "fixed' it keeps it fix for first comparative period where as other period keeps rolling.
The reportlet I added has two comparative period and in dashboard both funnels are displayed but in Report I can see both period calendar whereas in dashboard only one is showing up in calendar. second period data is there but calendar is missing.