Use COUNTIF if you need to sum values for a particular person or other criterion.

Use SUMIF if you need to sum values for a particular person or other criterion.

Usually you can view a formula in the Excel cell when cell is selected, but you can protect spreadsheet to hide formulas in some cells.

Because time stamps are represented as serial numbers, you can subtract the earlier time from the later time to get the difference.

Solver tool allows you to specify multiple adjustable cells and constraints on the values that the adjustable cells can have, generate a solution that maximizes or minimizes a particular worksheet cell, generate multiple solutions to a problem.

Goal seeking is a useful feature that works in conjunction with your formulas. If you know what a formula result should be, Excel can tell you which values of one or more input cells you need to produce that result.

When calculating the difference between two dates, you may want to exclude weekends and holidays.

A common type of date calculation determines the number of days between two dates.

You can easily count the number of unique values of the range using a simple formula.

If you would like to better understand how some of these complex array formulas work, consider using a handy tool - Formula Evaluator: