EMS , why filter doesnot work well for get-mailbox

i am writing a script to check mailbox, which created before someday.

user input '1/1/2013' for example , then  mailbox created before '1/1/2013' will show on screen.

see the image below. wrong result just come out when I use a  variable. so weird !

I know get-mailbox|where {xxxxx} can get the right answer, but '-filter' is more effective and faster.

so can anybody tell me why this filter cannot not work ?

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