day from date calculator

//This program will find the day from any date between 45BC to 9999AD.
//It uses the Julian Calendar between the dates 45BC and 1752AD. 
//It assumes 45BC (the year the Julian calendar was introduced) is a leap year.
//The year does not exist, 3BC, 2BC, 1BC, 1AD....
//Type 'config' to change the date the Gregorian calendar takes effect.

#include <iostream>
#include <string>
using namespace std;

int main()
{
//Collect Data
int england=1752;
start:
cout << "\nDDDDD           tt";                                     
cout << "\nDD  DD    aa aa tt      eee";
cout << "\nDD   DD  aa aaa tttt  ee   e";  
cout << "\nDD   DD aa  aaa tt    eeeee";       
cout << "\nDDDDDD   aaa aa  tttt  eeeee\n";   
    
cout << "\n CCCCC          lll                lll         tt";
cout << "\nCC    C   aa aa lll   cccc uu   uu lll   aa aa tt     oooo  rr rr";
cout << "\nCC       aa aaa lll cc     uu   uu lll  aa aaa tttt  oo  oo rrr  r";   
cout << "\nCC    C aa  aaa lll cc     uu   uu lll aa  aaa tt    oo  oo rr";
cout << "\n CCCCC   aaa aa lll  ccccc  uuuu u lll  aaa aa  tttt  oooo  rr";
cout << "\n\nby Rik Potts";   
    
    string start;
    cout << "\n\nEnter 'go' to begin or type 'config' for configuration: ";
    cin >> start;
    
    if (start!="go" && start!="config") 
{
    cout << start << " is not a valid entry. Please try again\n\n";
    goto start;
}
    
    if (start=="go") goto findday; else goto config;
    
    //Configuration     
    config:    
    cout << "\nPlease enter the date you want the Gregorian (modern) calendar to take effect.";
    cout << "\n\nDefault is 1752 which is when England began using the Gregorian";
    cout << "\ncalendar. The Gregorian calendar was first introduced in 1582";
    cout << "\nin Venice, France, Spain, Portugal and Netherlands. Germany in 1583";
    cout << ",\nPrussia and Denmark in 1700, Sweden in 1753 and Russia in 1918!\n\n";
    cout << "Please enter the date you want the Gregorian calendar to take effect: ";
    cin >> england;
    cout << "NOTE: You will have to set config each time you run the program.\n\n";
    
    //Find the day (day)   
findday:
    cout << "The Gregorian (modern) calendar will take effect from " << england;
    
    int day;
    cout << "\nPlease enter the day (##): " ;
    cin >> day;
    
    if (day>31 || day<1)
{   
    cout << "\n" << day << " is not a valid day!\nPlease try again\n\n"; 
    goto findday;
}
    
    //Find the month (month)...
month:
    string month;
    cout << "Please enter the Month (abc): ";
    cin >> month;
    
    if (month!="feb" && month!="aug" && month!="mar" && month!="nov" 
    && month!="jun" && month!="sep" && month!="dec" && month!="apr" 
    && month!="jul" && month!="jan" && month!="oct" && month!="may")
{   
    cout << "\n" << month << " is not a valid month!\nPlease try again...\n\n";
    goto month;
}
    
    if (month=="feb" && day>29) 
{
    cout << "\nIt is not possible to have " << day << " days in February!";
    cout << "\nPlease try again\n\n";
    goto findday;
}

     //Find the year (date)...
cent:
    cout << "\nNOTE: If you need a BC date you DO ";
    cout << "NOT need to use a minus sign (-)\n";
    
    //Get the century (cent)
    int cent;
    cout << "Please enter the first two digits of the year (##__): ";
    cin >> cent;
    
    if (cent>99) 
{
    cout << "\n" << cent << " is not a valid entry.\nYou must enter a number below 99.\n";
    goto cent;
}
    
    //Get the year (year)
year:
    int year;
    cout << "Please enter the last two digits of the year (__##): ";
    cin >> year;
    
    if (year>99 || year<0) 
{
    cout << "\n" << year << " is not a valid entry.\nYou must enter a number below 99\n\n";
    goto year;
}

    //make the full year (date)
    int date, numdate;
    date=(100*cent) + year;
    numdate=date;
    
    if (date==0)
{
    cout << "\nThe date 0000 is not valid because the Julian calendar uses 1BC";
    cout << " instead\n\n";
    goto cent;
}

    //Is it a negative or positive year?...
norp:
    string bc;
    cout << "Is the year BC? (y/n) : ";
    cin >> bc;
    
    if (bc!="y" && bc!="n") 
{
    cout << bc << " is not a valid option. Please try again...";
    goto norp;
}
    
    if (bc=="y") date=-date;
    
    if (date<=-46) 
{
    cout << "\nYou entered " << numdate << "BC. The Julian calendar was introduced ";
    cout << "in 45BC.";
    cout << "\nThis makes " << numdate << "BC invalid!\n";
    goto cent;
}
    
    int year2;
    year2=year/4;
    
//Process and Validate Data
    //Is the year a leap year?...
    //Is the year on the gregorian or julian calendar?
    int yl=1, nl=0;
    if (date>=england) goto gregorian;
    
    //Julian calendar
    int jleap;  
    if (date>0 && (date%4==0)) jleap=yl; else jleap=nl;
    if (date<0 && (date%4==-1)) jleap=yl; else jleap=nl;
    goto validation;
   
    //Gregorian calendar
gregorian:
    int gleap;
    gleap = ((date%4==0 && date%100!=0) || date % 400 == 0) ? yl : nl;

    //Is the Month and Day combination correct?
validation:
    if ((jleap==1 || gleap==1) && month=="feb" && day<=29) goto notfeb;
    
    if ((jleap!=1 || gleap!=1) && month=="feb" && day==29) 
{
    cout << "\nThe 29th February is not a valid date because " << date;
    cout << " \nisnt a leap year!\nPlease try again from the beginning\n\n"; 
    goto findday;
}
    
    if (month!="feb") goto notfeb;
    
    if (!(day<=28 && day>=1)) 
{
    cout << day << "\n is not a valid day!\nPlease try again\n\n"; 
    goto findday;
}

notfeb:
    if (month!="sep" && month!="apr" && month!="jun" && month!="nov") goto nsajn;
    
    if (!(day<=30 && day>=1)) 
{
    cout << day << "\n is not a valid day!\nPlease try again\n\n"; 
    goto findday;
}

nsajn:
    if (!(day<=31 && day>=1))
{
    cout << day << "\n is not a valid day!\nPlease try again\n\n"; 
    goto findday;
}

//Interpret Data
    //Calculate the Monthcode
    int monthcode;
    if (month=="aug") monthcode=0;
    if (month=="feb" || month=="mar" || month=="nov") monthcode=1;
    if (month=="jun") monthcode=2;
    if (month=="sep" || month=="dec") monthcode=3;
    if (month=="apr" || month=="jul") monthcode=4;
    if (month=="jan" || month=="oct") monthcode=5;
    if (month=="may") monthcode=6;
    
    //Which calendar to use?
    if (date>=england) goto gregoriansum;

    //Is the leap year compensation required?
    int ujleap, ugleap;
    ujleap=jleap;
    ugleap=gleap;
    if (month!="jan" && month!="feb") ujleap=nl;
    if (month!="jan" && month!="feb") ugleap=nl;
    
    //Julian Formula...
    //(Day + MonthCode + Year + Year/4 + 5 – Century – isLeapJanFeb?) % 7
    int jresult;
    if (date<0) jresult=(day + monthcode - year - (year2) + 5 - cent )%(-7);
    else
    jresult=(day + monthcode + year + (year2) + 5 - cent - ujleap)%7;
    
    goto weekday;
    
    //Gregorian Formula...
gregoriansum:
    //(Day + MonthCode + Year + Year/4 – 2×(Century%4) – isLeapJanFeb?) % 7
    int gresult;
    gresult=(day + monthcode + year + (year2) - (2*(cent%4)) - ugleap)%7;
    
    //Assign day code to word.
weekday:
    int weekday;
    string result;
    weekday = (date<england) ? jresult : gresult;
    if (weekday==0) result="Monday";
    if (weekday==1 || weekday==-6) result="Tuesday";
    if (weekday==2 || weekday==-5) result="Wednesday";
    if (weekday==3 || weekday==-4) result="Thursday";
    if (weekday==4 || weekday==-3) result="Friday";
    if (weekday==5 || weekday==-2) result="Saturday";
    if (weekday==6 || weekday==-1) result="Sunday";
    
//Print results
    if (date<0)
    cout << "\nThe day on " << day << " " << month << " " << numdate << "BC is a " 
    << result << "!\n";

    else 
    cout << "\nThe day on " << day << " " << month << " " << date << "AD is a " 
    << result << "!\n";

    if (date==england) 
{
    cout << "\nYou entered the year 1752!";
    cout << "\nThis was the year the Gregorian calendar was first ";
    cout << "\nproposed by Dr Aloysius Lilius!\n";
}
    
    if (date==(-45)) 
{
    cout << "\nYou entered the year 45BC!";
    cout << "\nThis was the year the Julian calendar was first ";
    cout << "\nintroduced by Julius Caesar!\n";
}
    
    if (date<0 && (jleap==1))
{
    cout << "The year " << numdate << "BC is a leap year!" << "\n\n";
    goto rerun;
}
    
    if (date>0 && (jleap==1 || gleap==1))
{   
    cout << "The year " << date << "AD is a leap year!" << "\n\n";
    goto rerun;
}
    
    if (date<0 && (jleap!=1))
{   
    cout << "The year " << numdate << "BC isnt a leap year!" << "\n\n";
    goto rerun;
}
    
    if (date>0 && (jleap==0 || gleap==0))
    cout << "The year " << date << "AD isnt a leap year!" << "\n\n"; 
    goto rerun;

rerun:
    cout << "Do you want to run the program again? (y/n): ";
    string startagain;
    cin >> startagain;
    cout << "\n";
    if (startagain=="y") goto findday; else goto close;
    
    close:
    return 0;
}