Question

In SQL 2008 why do I get an error when I do this:

select datediff (minute, '23:30','24:00') 

The error is :

Msg 241, Level 16, State 1, Line 1
Conversion failed when converting date and/or time from character string.

Even if I were to change 24:00 to 00:00 why does the result say -1410?

What are the alternatives that I could use for this please?

Answer 1

If you are looking for the difference between ’23:30′ and ’24:00′ – which technically is the next day, then you will use

select datediff (minute, '2012-07-10 23:30','2012-07-11 00:00') 

When you use:

select datediff (minute, '23:30','00:00') 

the ’00:00′ is being interpreted as the same day that is why you are getting -1410. There are a total of 1440 minutes in the day minus 30 minutes as the difference. The DATEDIFF function is:

DATEDIFF ( datepart , startdate , enddate )

If your end date ’00:00′ is before your start date then you will get a negative number.

Edit:

Based on your comment you will use the following for your table, then use a CASE statement around the endtime to add a day to it:

select datediff(minute
                , starttime
                , case 
                when endtime = '00:00' 
                then dateadd(d, 1, endtime)
                else endtime END)
from yourtable

See SQL Fiddle with Demo