JulianDate basically counts the number of days since Jan 1, 4713 BC 12:00 UTC in the Julian calendar. However, that is a bit hard to understand since it is so long ago, uses a different calendar, and you must take the transition form BC to AD into account.
So we use Reduced JulianDate, which is another official standard, related by $\text{Reduced JD} = \text{JD} - 2400000$. This counts the number of days since Nov 16, 1858 12:00 TT (Terrestrial Time) in the (usual) Gregorian calendar.
Then we may add 45.5 days to reach Jan 1, 1859 00:00 UTC. We can add 51499 more days to reach Jan 1, 2000 00:00 (Terrestrial Time), the beginning of a new 400-years cycle.
We may now compute:
$$\text{JD} = 2451544.5 + 365Y + \lfloor 0.25Y \rfloor - \lfloor 0.01Y \rfloor + \lfloor 0.0025Y \rfloor +A \\ +D + \tfrac1{24}H+ \tfrac1{1440}M + \tfrac1{86400}S$$
Where:
- $Y$ is the current Gregorian year minus 2000
- $\lfloor x \rfloor$ is the floor function, the largest integer smaller than $x$.
- $D$ gives the current day number. This is 1 on Jan 1, 32 on Feb 1, etc.
- $H$ is the current hour in Terrestrial Time.
- $M$ is the current minute in Terrestrial Time.
- $S$ is the current second in Terrestrial Time, including milliseconds, etc.
- $A$ is a correction factor. If the year is a leap year, it is -1, otherwise it is zero.
This is just a formula that works. There are many other formulas that would work, but I picked this one because it was the easiest to explain.
The difference of 69 seconds is caused by the difference between Terrestrial Time and UTC, currently 68.184 seconds. This is because a diffence of 36 seconds in leap seconds and a historical offset of 32.184 seconds.