sql - Time difference in minutes between two times -

been struggling difference in minutes between 2 times. 1 time comes field of data type time in sql server. other value string added sql query. i'm running in ssrs 2008 r2 looks this:

datediff(mi, convert(time, '23:59:59'), _travel.start_time) time_difference 

i error: conversion failed when converting date and/or time character string

i've tried casting '23:59:59' time type 'as time' similar error. have tried declaring variable ssrs won't allow it. try give similar error. time 23:59:59 same, _travel.start_time whatever in field in db. output _travel.start_time in format 00:00:00

any ideas how can difference in minutes between 23:59:59 , db value? ta

added:

can't believe it. _travel.start_time varchar not time. have tried

datediff(convert(time, '23:59:59'), convert(time, _travel.start_time))  

but still getting same error. however, have noticed there nulls/empty fields in _travel.start_time field

i see no issue have, unless ssrs issue or _travel.start_time not of type time.

see this: sql fiddle

sample sql:

-- sample trimming time portion off datetime select datediff(mi, convert(time, '23:59:59'),         ltrim(right(convert(varchar(20), getdate(), 100), 7))) time_difference  -- sample time conversion of datetime select  datediff(mi, convert(time, '23:59:59'),           convert(time, getdate())) time_difference 

in addition, have tried converting db value:

select datediff(mi, convert(time, '23:59:59'),         convert(time, _travel.start_time)) time_difference