converting datetime to varchar wrong date

Question

Why am I getting this: 1905-06-28 00:00:00.000 as a result of the following query.

declare @report_date datetime
set @report_date = 2013-1-8
select Convert(varchar(24),@report_date, 21)

Answer 1

2013-1-8 = 2004 (parsed as days from 1900-01-01)  

set quotes