ios - How to calculate NStimeZone differeces -

hi trying sync time server time. in india(ist) getting time zone , converting gmt format , calculating timestamp , finding delta value server time , using timestamp .. thats working fine.. me..

now changing time zone , calculating timeinterval in between existing tiemzone , new timezone , appending delta value not getting getting time interval 19800

    nsdate* sourcedate1 = [nsdate date];     nstimezone* sourcetimezone1 = [nstimezone timezonewithabbreviation:@"asia/clacutta"];     nstimezone* destinationtimezone1 = [nstimezone localtimezone];             nsinteger sourcegmtoffset1 = [sourcetimezone1 secondsfromgmtfordate:sourcedate1];     nsinteger destinationgmtoffset1= [destinationtimezone1 secondsfromgmtfordate:sourcedate1];     nstimeinterval interval1 = destinationgmtoffset1 - sourcegmtoffset1;     nsdate* destinationdate = [[nsdate alloc] initwithtimeinterval:interval1 sincedate:sourcedate1]; 

long delt=interval1+delta;

i not getting current result .. trying right way? or other thing need change