How can I convert C++ code to C?

#include <bits/stdc++.h>
using namespace std;


//for time I made a class
class DepartureTime
{   
int hh;
int mm;
int ss;

public:
DepartureTime()
{
hh =0; mm = 0; ss=0;
}

DepartureTime(int hr, int min, int sec)
{
hh = hr;
mm = min;
ss = sec;
}

//return as int
int getTime()
{
return hh*10000 + mm*100 + ss;
}

//return as string to display
string toString()
{
string a =to_string(hh) + " : " +to_string(mm) +" : " + to_string(ss);
return a;
}
};

//strucy
struct planes
{
int planeID;
int routeID;
DepartureTime time; // hr:mm:ss in 24 hour format
planes* plane; //self referential structure
};

//fill planeid and route id
void function1(planes array[], int size)
{
for(int i=0; i<size; i++)
{
//rand gives a randome number, take mod by 900 to get a number less than 900 and add 100 to get a number from
// 0 t0 1000
int v = rand() % 900 + 100;
int v2 = rand() %1000 + 1000;
array[i].planeID = v;
array[i].routeID = v2;
}
};


//initialise with random time
void function2(planes array[], int size)
{
for(int i=0; i<size; i++)
{
int hr = rand() %24;
int min = rand() %60;
int sec = rand() %60;

DepartureTime t(hr, min, sec);
array[i].time = t;
}
}


//display
void function3(planes array[], int size)
{
cout<<"Plane ID\t Route ID\t Departure DepartureTime\n";
for(int i =0; i<size; i++)
{
cout<<array[i].planeID<<"\t\t"<<array[i].routeID<<"\t\t"<<array[i].time.toString()<<endl;
}
}


//sort in descending using selection sort
void function4(planes array[], int size)
{
for(int i=0; i<size; i++)
{
int min =i;
for(int j=i+1; j<size; j++)
{
if(array[i].time.getTime() < array[j].time.getTime())
{
planes temp = array[i];
array[i] = array[j];
array[j] = temp;
}
}
}

}


void function5(stack<planes>* s)
{
//last element of array will be removed, which is done by size-- itself
(*s).pop();   
}

void function6(queue<planes>* q, planes array[], int size)
{
(*q).pop();
//also remove from first element of array
for(int i=1; i<size; i++)
{
array[i-1] = array[i];
}
}

int main()
{
planes array[15];
int size =15;
function1(array, size);
function2(array, size);

function4(array, size);
function3(array, size);

//implement stack and queue
// in stack last in first out, use stack to take off plane with earliest departure
//in queue first in first out, use stack to take off plane with last scheduled time
stack<planes> s;
for(int i=0; i<size; i++)
{
//no planes struct exist before 0
if(i != 0)
array[i].plane = &(array[i-1]);

s.push(array[i]);
}

queue<planes> q;
for(int i=0; i<size; i++)
{
q.push(array[i]);
}

//in function 5 we pass stack by address to show the changes in main fucntion and decrease size to reduce plane by 1
function5(&s);
size--;
cout<<endl;
cout<<"After a plane takes off from schedule\n";
cout<<endl;
//DISPLAy again
function3(array, size);

cout<<endl;
cout<<"After taking off the last schedules plane\n";
cout<<endl;
function6(&q, array, size);
size--;
cout<<endl;
//DISPLAy again
function3(array, size);
}