The beginning of the khums fiscal year is the day one makes a profit, even when he was a child. Based on this, it does not matter when you started your work.
Since you do not know this day, you should set the khums fiscal year by reconciliation (Musalahah) with the Marjaâ€™ or his agent.
<p style="text-align: left;">The reason behind the waiting period is not necessarily identifying the father if the woman is pregnant. If this was the reason, then it would have been sufficient for Almighty Allah the Divine Legislator to consider one menstrual cycle as a waiting period and not any longer duration. Almighty Allah knows the actual reason behind such legislation.<br /> Therefore, the iddah must be observed even if contraception was used.</p>