Transact SQL :: Accumulate Values Based On Time For Each Day

I have a table Mytable which has two fields (Date, Value) and record is based on every hour like this:

Date Value
1999-01-01 00:00:00 10
1999-01-01 01:00:00 8
1999-01-01 02:00:00 6
...
1999-01-01 23:00:00 12
1999-01-02 00:00:00 9
1999-01-02 01:00:00 5
...
1999-01-02 23:00:00 2

How can I use a query to get accumulate value for the day? the result should look like this:

Date Value Accumulated_Value
1999-01-01 00:00:00 10 10
1999-01-01 01:00:00 8 18
1999-01-01 02:00:00 6  24...
1999-01-01 23:00:00 12 36
1999-01-02 00:00:00 9 9
1999-01-02 01:00:00 5 14...
1999-01-02 23:00:00 2 16

The accumulated value should be based on every hour for the day and then repeat for the second day and so on.

What is the best way to do this? I am using SQL 2008