Slow query due to variable

Hello everyone,

this query is driving me crazy The problem is that when I'm in a server and execute a query against another server, it works well when I use hardcoded values but turns terrible slow when I use a variable. I've looked everywhere but haven't got the right answer. This problem exist in queries and Stored Procedures. I've tried this with parameters instead of variables and the problem persists.

The weird thing is that two days ago I was able to execute the query qith the variable without problems.

Any insight will be welcome

Thanks

USE DataBaseInServerA

Declare @date smalldatetime
Set @date = ‘1987-03-15’
Select * from ServerB.asd.dbo.person
Where birthday = '1987-03-15' --- < This works perfectly

Select * from ServerB.asd.dbo.person
Where birthday = @date --- < This takes toooo long