I need to display time in two cells in different format

a1 = indian standard time
b1=corresponding GMT

example: a1=06:00 then b1=00:30 (IST = +5:30 GMT)
a1=02:00 then b1=20:30

Is there a formula which will do the trick?

Furthermore, I want to enter IST in cell A1 by simply entering 0600 or 0200,
can this be automatically
formatted to display 06:00 or 02:00?