Cell preservation or previous value.
 

Is there a function that will allow you to maintain the current value of
a cell or retrieve the previous value of a cell? Consider the following
equation:

IF(Day(Today()) = 23, perform calculation, perform no calculation and
maintain the value from the previous calculation)

This could also be done with a function that allow you to assign values to
other cells. If a function in cell C3 assigns a value or the results from a
caculation to Cell H5.