SQL GroupBy DateTime over X minutes

It seems to me surprisingly hard to do in SQL, but you need to calculate the time rounded down to the last 5 minute amount, then group by that amount. and to do that I had to go through a few steps. How about I make a standard function for you, to help keep your code cleaner?

this is for 5 minute segments, I have datepart(minute, d.Time)) / 5) * 5, you can change both the 5's to any other whole number or make it into an integer variable - it takes the whole day part of your date (can be done in higher SQL versions by casting to date if you wanted) and then to that date, it rounds the number of minutes into the day to the lower 5, then adds them on

 select d.Data,
       dateadd(minute, 
                 floor((datepart(hour,d.Time) * 60 
                              + datepart(minute, d.Time)) / 5) * 5,          
                 cast(convert(nvarchar(8), d.Time, 112) as datetime)
               ),
  avg(d.Count) as [CountVal]
from data d

GROUP BY dateadd(minute, 
                 floor((datepart(hour,d.Time) * 60 
                              + datepart(minute, d.Time)) / 5) * 5,          
                 cast(convert(nvarchar(8), d.Time, 112) as datetime)
                 )

here it is done by a function that you have to first create

    -- ================================================
-- Template generated from Template Explorer using:
-- Create Scalar Function (New Menu).SQL
--
-- Use the Specify Values for Template Parameters 
-- command (Ctrl-Shift-M) to fill in the parameter 
-- values below.
--
-- This block of comments will not be included in
-- the definition of the function.
-- ================================================
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Author:      <Author,,Name>
-- Create date: <Create Date, ,>
-- Description: <Description, ,>
-- =============================================
CREATE FUNCTION fn_DateTime_n_MinuteSector
(
    -- Add the parameters for the function here
    @PDATE datetime,
    @PNMINS int
)
RETURNS datetime
AS
BEGIN

    DECLARE @WDATE datetime;
    DECLARE @DAYMINS int;
    DECLARE @ROUNDMINS int;

    --calculate Whole Date and number of complete minutes elapsed since start of date
    SELECT @WDATE = cast(convert(nvarchar(8), @PDATE, 112) as datetime), 
           @DAYMINS = datepart(hour,@PDATE) * 60 + datepart(minute, @PDATE);

    --round down to n minutes in day
    SELECT @ROUNDMINS = FLOOR(@DAYMINS / @PNMINS) * @PNMINS;

    --create rounded date by adding minutes on to date part
    RETURN dateadd(minute , @ROUNDMINS, @WDATE) ;


END
GO

then to use your function in a dbo.fn_DateTime_n_MinuteSector(d.time, 15)