How to give specific default date with expression in SSRS

Hi,

I have to pass some specific kind of dates in "Default Values" section of parameter.

1st is whenever SSRS report run for any year in default date I want to pass value 1/1/(Current Year), for this I tried many way like , = "1/1/" + DatePart("Y",Now())  also = "1/1/" + Year(Now()) but it is giving me error. When I tried both part individually "1/1/" and  DatePart("Y",Now()) , it is working but when I join them with + sign it is giving me error that "input string is not in correct format"

2nd I want to set up like when package run in month of April, it takes default date as "(Previous Month, which is May)/1/(current Year)" so if I am running report on 4/5/2013 then it in default date it should take 3/1/2013.

Please help me with the expression. Thanks in advance. 

Vicky