C.I.T.O.K.A.T.E.

Here's a methodology --

number_of_days = (2005 - 1974 * 365) + ((2006 - 1974) / 4) - (31 + 28 + 31 + 30 + 31 + 16) + (31)
/* How many days old are you right now? This gives you the number of days since Jan 1 1974 + plus the number of leap years - the Julian number of days until you were born + the current number of Julian days of the current year */

But what you really want is when you'll be a billion seconds, so lets see how many days a billion seconds is:

minutes = 1,000,000,000 / 60 = 16,666,666 min 40 secs
hours = 16,666,666 / 60 = 277,777 hr 46 min 40 sec
days = 277,777 / 24 = 11,574 days 1 hr 46 min 40 sec
years = 11,574 / 365 = 31 years, 259 days, 1 hr 46min 40 sec

So 259 days from June 17 (Julian date is 31 + 28 + 31 + 30 + 31 + 17 = 158)
Day offset = 365 - 158 = 207 days
Day offset 2 = 259 - 207 = 42 days

Your Billion second birthtime = Feb 11th, 2006 at 03:46:40

Someone may want to check my math/assumptions, as I may be a day off, and I don't have time to check my work.