select

DT,

SecondsSince_19000101 =

-- seconds for whole days

(datediff(dd,0,a.DT)*000000000086400)+

-- seconds since start of day

datediff(ss,dateadd(dd,datediff(dd,0,a.DT),0),a.DT)

from

( -- test data

select DT = getdate()union all

select DT = '99991231 23:59:59.997'union all

select DT = '18991231 12:01:01.997'union all

select DT = '17530101 00:00:00.000'

) a

order by

a.DT

Results:

DT SecondsSince_19000101

----------------------- ---------------------

1753-01-01 00:00:00.000 -4638816000

1899-12-31 12:01:01.997 -43139

2013-05-08 15:07:18.380 3577014438

9999-12-31 23:59:59.997 255611289599

Edited: Replaced 86000 with correct value of 86400 for number of seconds in a day.