getting julian date in unity?

First of all Day Number number is not Julian Day

Day Number is very basic, DateTime class of C# provides you with Day Of Year which is total number of days since the start of current year and leap-year phenomenon is handled within the class, see the MSDN Documentation.

All you need to do is add System library to your code

 using UnityEngine;
 using System.Collections;
 using System;
 
 public class myClass: MonoBehaviour {
 
 void Start () {
     Debug.Log(DateTime.Now.DayOfYear.toString());
 }
 
 void Update(){
 }

 }

And as far as Julian Day is concerned, it is number of total days since the beginning of Julian Period.

Julian Period started at January 1, 4713 BC 12:00:00 hrs. So you need to calculate total number of days since Jan-1,4713 BC for that take the difference between the Year (Current or any) and 4713 BC. Count total number of leap years within that period.

 Julian Day = ( Difference of Years * 365 ) + NumberOfLeapYearsInThisPeriod

What this will do is add 1 day for all leap years.

Julian Date is more precise than Julian Day and is in decimal representing how much of a day has passed, remember that Julian Period started at January 1, 4713 BC 12:00:00 hrs at noon, meaning Julian day starts & ends on mid day.

For today May-11,2014 Julian Day is 2456788. Which is true before noon 12:00 hrs after that the Julian Day is +1 which is 2456789. So you need to write algo to which considers 24 hrs as 1, 12 hours = 0.5. And so and so forth calculations down to milliseconds.

For May 11, 2014 11:00:00 hrs the Julian Date is 2456788.95833

Supposing we already have Julian Day, heres the code for Julian Date

     int julianDay; //let's suppose we have value of Julian Day here
     float julianDate;
     now = DateTime.Now;
     int mSecs = (((now.Hour * 60) + now.Minute) * 60 + now.Second) * 1000 + now.Millisecond;
     julianDate = (mSecs < 43200000) ? --julianDay + (mSecs/43200000) : julianDay + (mSecs-43200000)/86400000;

Explanation is a day has 86400000 milliseconds and at mid day 43200000 millisecods. So by dividing time in milliseconds by 86400000 or 43200000 I get decimal portion of day or half day respectively.