» Placing the Previous Payment Date beside Each Pay Date in a List

Problem:

Columns A & B contain Client ID's and their recent payment dates.
We want to add the date of the previous payment from the same client next to each entry.

Solution:

Use the INDEX, MATCH, MIN, and IF functions in the following Array formula:
{=INDEX($B$2:$B$10,MATCH(MIN(IF(((B2-$B$2:$B$10)>0)*(A2=$A$2:$A$10),(B2-$B$2:$B$10))),B2-$B$2:$B$10,0))}
Enter the formula in cell C2, and drag down the column.
If a previous payment date does not exist for a particular client, the date of the current pay date will be displayed in both columns.