You can simply test the inverse :

!date1.after(date2)

You can always convert a strict order check to a non-strict check in this manner. Since mathematically :

a > b ⇔ ¬ (a ≤ b)

Just wanted to chime in:

• Nobody mentioned this, but isValidDate() to determine whether 1 date is earlier than another date is frankly terrible. Call it isEarlierDate()
• Where are the variables? Are those variables all defined outside of this function. That does not look good.

• If there is one thing you learn today it is that this

  if(day<=dayCurr)
      return true;
  else
      return false;
  

  should always be written as

  return day<=dayCurr
  

• As per @Heslacher, write out your variable names

This is my over-commented counter-proposal:

function isEarlierDate(){

  return year < currentYear || //Last year is in the past
         year == currentYear && month < currentMonth || //This year, earlier month is in the past
         year == currentYear && month == currentMonth && day < currentDay || //..
         false; //This is clearly not an earlier date
}