mysql converting from UTC to IST

IST is 5.30 hours ahead of UTC, so when 13th starts in IST i.e. 2015-03-13 : 00:00:00 its 2015-03-12 18:30:00 in UTC

mysql> select convert_tz('2015-03-13T00:00:00+00:00','+00:00','+05:30') ;
+-----------------------------------------------------------+
| convert_tz('2015-03-13T00:00:00+00:00','+00:00','+05:30') |
+-----------------------------------------------------------+
| 2015-03-12 18:30:00                                       |
+-----------------------------------------------------------+
1 row in set, 1 warning (0.00 sec)

And when 13 ends in IST i.e. 2015-03-13 : 23:59:59 its 2015-03-13 18:29:59 in UTC

mysql> select convert_tz('2015-03-13T23:59:59+00:00','+00:00','+05:30') ;
+-----------------------------------------------------------+
| convert_tz('2015-03-13T23:59:59+00:00','+00:00','+05:30') |
+-----------------------------------------------------------+
| 2015-03-13 18:29:59                                       |
+-----------------------------------------------------------+

So yo get the data in IST for 13th you will need to search data within this range of dates.

So the condition would be as below -

s.created_at 
between convert_tz('2015-03-13T00:00:00+00:00','+00:00','+05:30')
and convert_tz('2015-03-13T23:59:59+00:00','+00:00','+05:30');

and since you are doing conversion at the time of select so it will return all 13th data.