Often deveopers confuse themselves between Timestamp and Datetime datatypes. But they are entirely different datatypes. Although the name has Time, the Timestamp datatype has nothing to do anything with date or time. Also you cannot explicitely add/update that column. It gets updated whenever data are added or updated in the table.

Consider the following example

declare @t table(names varchar(100), uid timestamp)
insert into @t (names) 
select 'test1' union all
select 'test2'

select * from @t 

Which produces the following result

names                    uid
------------------------ ------------------
test1                    0x00000000000007D8
test2                    0x00000000000007D9

Now update the first record and see the result

update @t 
set names='test100' where names='test1'

select * from @t 

names                    uid
------------------------ ------------------
test1                    0x00000000000007DA
test2                    0x00000000000007D9

As you see values of Timestamp column is automatically updated whenever data are added or modified. So do not confuse with Datetime datatype. Timestamp datatype does not represent any date ot time values.