when Google was just founded, or Zack just graduated from Harvard, or on the day when Steve job returned to Apple. This is because probabilitiy wise, you could very easily invest in Yahoo, MySpace, or Sun Microsystem instead.
The best time to invest in Google is when it becomes quite clear Yahoo is yesterday's news and monetization through Ads is the future, to invest in Facebook when it becomes clear social media is no longer a college hobby but is used by 100M users and growing into billions, to invest in Apple when Steve accnouced iPhone in 2007.
This is why now is the best time to invest in Tesla
