Description:class to calculate the difference between the two dates passed and return the difference in one(seconds, minutes, hours, days) of the formats specified
This constructor sets the values of these variables and calls the checkValid function to check the validity of the dates passed to the function.
calculates the difference between the two dates and returns the difference in the specified format
Pre-requisite for using the class:
Second date should be greater than the first date
Dates passed to the function should be valid