This post is part of a series on understanding the meaning behind the values contained in a Two Line Element set, or Keplerian Elements. The main article is here.
Calculation of the Julian Day Number (JDN)
Julian day numbers are regularly used in astronomy instead of a date.
Julian day numbers are simply the count of days that have elapse since noon on January 1, 4713 BC Greenwich time using the Julian proleptic calendar.
We generally use the Gregorian calendar. Calculation to a Julian date is a little cryptic as the two calendars have different rules for calculating leap years.

Julian Calendar
 Every 4 years

Gregorian Calendar
 Every 4 years except when the year is divisble by 100
 But ALWAYS every 400 years
To calculate the value, we first get the Julian date of the year, then add on the days in the year followed by the fraction of the day.
Some astronomers like to use a Modified Julian Date. This is simply the Julian Date with 2,400,000.5 subtracted from it making the number smaller and based on midnight rather than noon UTC.
Here is some C# which will calculate the following:
 Julian Day from a DateTime
 Julian Day for a year
 Julian Day from a Satellite Epoch
 Converting between modified and standard Julian dates
double getJulianDay_DateTime(DateTime date) { //Calculate the day of the year and the double dDay = date.DayOfYear; dDay += Convert.ToDouble(date.Hour) / 24; dDay += Convert.ToDouble(date.Minute) / 1440; dDay += Convert.ToDouble(date.Second) / 86400; return dDay + getJulianDay_Year(date.Year); } double getJulianDay_Year(int year) { double dYear = year  1; double A = Math.Floor(dYear / 100); double B = 2  A + Math.Floor(A / 4); //The use of 30.600000000000001 is to correct for floating point rounding problems double dResult = Math.Floor(365.25 * dYear) + 1721422.9 + B; return dResult; } double getJulianDay_SatEpoch(int year, double dSatelliteEpoch) { //Tidy up the year and put it into the correct century year = year % 100; if (year < 57) year += 2000; else year += 1900; double dResult = getJulianDay_Year(year); dResult += dSatelliteEpoch; return dResult; } double getModifiedJulian_Julian(double dJulian) { return dJulian  2400000.5; } double getJulian_ModifiedJulian(double dModifiedJulian) { return dModifiedJulian + 2400000.5; } 
I think you might have a typo in the following line?
“double dResult = Math.Floor(365.25 * dYear) + 1721422.9 + B;”
I believe that the number should be 1721422.5. Thanks!