date ranges t/sql

for tide digest i am operative i need relapse an sum news formed date ranges.

i have 3 forms reports, yearly, monthly daily.

to support returning news i need duty relapse sub-ranges datetimes, within large range.

so instance i daily ranges between '2006-01-01 11:10:00' '2006-01-05 08:00:00' i design following results.

select * 
from dbo.fngetdateranges('d', '2006-01-01 11:10:00', '2006-01-05 08:00:00')  

2006-01-01 11:10:00.000,    2006-01-02 00:00:00.000
2006-01-02 00:00:00.000,    2006-01-03 00:00:00.000
2006-01-03 00:00:00.000,    2006-01-04 00:00:00.000
2006-01-04 00:00:00.000,    2006-01-05 00:00:00.000
2006-01-05 00:00:00.000,    2006-01-05 08:00:00.000

for yearly operation '2006-01-01 11:10:00' '2009-05-05 08:00:00', i expect.

select * 
from dbo.fngetdateranges('y', '2006-01-01 11:10:00', '2009-05-05 08:00:00') 

2006-01-01 11:10:00.000,    2007-01-01 00:00:00.000
2007-01-01 00:00:00.000,    2008-01-01 00:00:00.000
2008-01-01 00:00:00.000,    2009-01-01 00:00:00.000
2009-01-01 00:00:00.000,    2009-05-05 08:00:00.000

how i exercise function?