Calculating the no. of day/weeks/months/year between dates:

Calculating the difference between each pair of dates listed in columns
A & B.

Solution:

To calculate the difference in days, use the DATEDIF function as shown in the following formula:
=DATEDIF(A2,B2,"d")

To calculate the difference in weeks, use the INT function as shown in the following formula:
=INT((B2-A2)/7)

To calculate the difference in months, use the DATEDIF function as shown in the following formula:
=DATEDIF(A2,B2,"m")

To calculate the difference in years, use one of the following two solutions:
Use the DATEDIF function as shown in the following formula:
=DATEDIF(A2,B2,"y")