If you look at the gem documentation, you will see that the :after and :before params take in a date in the format of YYYY-MM-DD.
From the gem Readme:
gmail.inbox.count(:after => Date.parse("2010-02-20"), :before => Date.parse("2010-03-20"))
gmail.inbox.count(:on => Date.parse("2010-04-15"))
Your code is passing in YYYY-DD-MM which is likely causing the error.
Edit
When you call strftime on a datetime object, it doesn't modify the object - only returns the string notation based on format you give.
As a result, the Date.parse(t) is still getting Time.now was a parameter.
Try this:
t = Time.now.strftime("%Y-%m-%d")
Date.parse(t)