Julian (Old Style)

Julian Date = 367K - <(7(K+<(M+9)/12>))/4> + <(275M)/9> + L + 1721013.5 + UT/24 - 0.5sign(100K+M-190002.5) + 0.5

K=Year (YYYY)
L=Day
M=Month

"< >" extracts the integral part of a number
For e.g. 3.999 -> 3, 1.002 -> 1

And the "Sign" function extracts the algebraic sign from a number. Its either -1 or 1.
For e.g. sign(2548484)= 1 and sign(-2151851)= -1

UT refers to the Universal Time. M'sia is UT+8. For other time zones you can refer to http://www.timeanddate.com

For e.g. 27th July 1986 5am in Julian would be 2446624

728862 - 3491 + 213 + 26 + 1721013.5 + 21/24 -0.5(1) + 0.5 = ~2446624
Strictly speaking the HD would be 2446624.375, but the decimal point makes things messy.

Note: Although this example is calculating the 27th, the day used in the equation could also be the 26th or the 28th depending on the UT at Greenwich. Same goes for the time in the equation.
Reference: http://aa.usno.navy.mil/faq/docs/JD_Formula.html