Time arithmatic #include<stdio.h> #include<conio.h> typedef struct { int hh,mm,ss; }time; time add(time f,time s); time sub(time f,time s); time input(void); void main() { time f,s,ans; int ch; clrscr(); do { printf("n <1> enter first operand "); printf("n <2> enter second operand "); printf("n <3> time addition "); printf("n <4> time subtraction "); printf("n <5> Exit nn"); do { printf("enter your choice "); scanf("%d",&ch); }while(ch<1 || ch>5); switch (ch) { case 1: printf("n enter the time(hh mm ss) "); f=input(); break; case 2: printf("n enter the time(hh mm ss) "); s=input(); break; case 3: ans=add(f,s); printf("n time addition is %d %d %d",ans.hh,ans.mm,ans.ss); break; case 4: ans=sub(f,s); printf("n time subtraction is %d %d %d",ans.hh,ans.mm,ans.ss); break; } }while(ch!=5); } time input(void) { time temp; do { printf("n enter the hours : "); scanf("%d",&temp.hh); } while(temp.hh<0 || temp.hh>12); do { printf("n enter the minutes : "); scanf("%d",&temp.mm); } while(temp.mm<0 || temp.mm>=60); do { printf("n enter the seconds : "); scanf("%d",&temp.ss); } while(temp.ss<0 || temp.ss>=60); return temp; } time add(time x,time y) { time a; if(x.ss+y.ss>60) ++x.mm,a.ss=(x.ss+y.ss)%60; else a.ss=x.ss+y.ss; if(x.mm+y.mm>60) ++x.hh,a.mm=(x.mm+y.mm)%60; else a.mm=x.mm+y.mm; if(x.hh+y.hh>12) a.hh=(x.hh+y.hh)%12; else a.hh=x.hh+y.hh; if(a.hh==0) a.hh=12; return a; } time sub(time x,time y) { time a; if(x.ss-y.ss<0) --x.mm,a.ss=60+x.ss-y.ss; else a.ss=x.ss-y.ss; if(x.mm-y.mm<0) --x.hh,a.mm=60+x.mm-y.mm; else a.mm=x.mm-y.mm; if(x.hh-y.hh<0) a.hh=12+x.hh-y.hh; else a.hh=x.hh-y.hh; if(a.hh==0) a.hh=12; return a; } It will be more readable at the link: http://www.cplusplus.happycodings.com/Algorithms/code30.html