I also had an issue with usp_Main_GetSessionInfo due to the max value for milliseconds in the DateDiff function being less than 24 Days (etc). Therefore, to maintain a good degree of accuracy still (and having had the problem before), I rewrote it like so :-

USE [msdb]

GO

ALTER PROCEDURE [MS_PerfDashboard].[usp_Main_GetSessionInfo]

AS

BEGIN

SELECT COUNT(*) AS num_sessions,

SUM(s.total_elapsed_time) AS total_elapsed_time,

SUM(s.cpu_time) AS cpu_time,

SUM(s.total_elapsed_time) - SUM(s.cpu_time) AS wait_time,

SUM(s.login_time) - SUM(s.total_elapsed_time) AS idle_connection_time,

cache_hit_ratio=(CASE WHEN SUM(s.logical_reads) > 0 THEN (SUM(s.logical_reads) - ISNULL(SUM(s.reads),0)) / CONVERT(FLOAT,SUM(s.logical_reads))

ELSE NULL END)

FROM (SELECT session_id,

CONVERT(BIGINT,total_elapsed_time) AS total_elapsed_time,

CONVERT(BIGINT,cpu_time) AS cpu_time,

CONVERT(BIGINT,DATEDIFF(ss,login_time,GETDATE()))*1000 AS login_time,

logical_reads, reads

FROM sys.dm_exec_sessions

WHERE is_user_process = 0x1

AND login_timeDATEADD(minute,-35791,GETDATE())) s

END