If i got it right ...

Suppose there's date range lookup table with N dates

create table #RangeLookup (

dt date

)

declare @strtDate date = '20140101'

declare @step int = 30

declare @N int = 20

insert #RangeLookup

select top (@N) dateadd(dd, @step*(row_number() over (order by (select null))-1), @strtDate)

from sys.all_columns

Step is not supposed to be always fixed as it is in the above example.

Now one can build a view or cte kind of

;with c1 as (

select dt, rn= row_number() over (order by dt)

from #RangeLookup

),

RangeLookup2 as ( -- N-1 intervals with tags

select dStart = a.dt

, dEnd = dateadd (dd, -1, b.dt)

, tag = datediff (dd, (select top 1 dt from #RangeLookup order by dt), b.dt)

from c1 a

join c1 b on a.rn= b.rn-1

)

select * from RangeLookup2

Then just compare myDate in question against interval to get the tag.

Rgds

Serg