DECLARE @GLR        TABLE
(
     FDSTART    smalldatetime
     , FDEND    smalldatetime
     , fnnumber int
)

DECLARE @T  TABLE
(
    SingleDate      smalldatetime
    ,SomeNumber     int
    ,NiceNewNumber  int
)

Insert into @T ( SingleDate     ,SomeNumber) values ('jan 1 2010', 1);
insert into @T ( SingleDate     ,SomeNumber) values ('jan 2 2010', 5);
insert into @T ( SingleDate     ,SomeNumber) values ('jan 9 2010', 8);
insert into @T ( SingleDate     ,SomeNumber) values ('jan 18 2010', 3);
insert into @T ( SingleDate     ,SomeNumber) values ('jan 20 2010', 4);

insert into @GLR(FDSTART  , FDEND  , fnnumber) values ('Jan 1 2010', 'Jan 8 2010', 11);
insert into @GLR(FDSTART  , FDEND  , fnnumber) values ('Jan 9 2010', 'Jan 14 2010', 55);
insert into @GLR(FDSTART  , FDEND  , fnnumber) values ('Jan 17 2010', 'Jan 30 2010', 44);

/* some simple updating with the join being a between op.  */
UPDATE      T
SET         NiceNewNumber   =  G.fnnumber

FROM        @T AS T
INNER JOIN  @GLR AS G
ON          T.SingleDate BETWEEN G.FDSTART  AND G.FDEND


select * from @T