Conversion failed when converting date and/or time from character string SQL Server

Conversion failed when converting date and/or time from character string
SQL Server

I am Surprised, While i copy and paste 'Mon, 26 Aug 2013 5:32:44 GMT' in
database table field of type DateTime in Editable mode it update that cell
but while i am Executing query Like
declare @Manoj date
Set @Manoj = 'Mon, 26 Aug 2013 5:32:44 GMT'
update NK_News set PubDate =@Manoj where ID= 70
here PubDate is DateTime field
It generate error as Conversion failed when converting date and/or time
from character string
Is there any way to find what exactly Query run at Editable Mode in SQL
Server Management Studio.