To count cells that contain only odd numbers, you can use a formula based on the SUMPRODUCT function together with the MOD function.
In the example, the formula in cell E6 is:
=SUMPRODUCT(--(MOD(rng,2)=1))
This...

To sum every nth column, you can use a formula based on the SUMPRODUCT, MOD, and COLUMN functions.
In the example shown, the formula in L5 is:
=SUMPRODUCT(--(MOD(COLUMN(B5:J5)-COLUMN(B5)+1,K5)=0),B5:J5)
How this...

To get the duration between two times in as decimal hour (i.e. 3 hrs, 4.5 hrs, 8 hrs, etc.) you can use a formula based on the MOD function.
In the example shown, the formula in D5 is:
=MOD(B2-A2,1)*24
Excel hours...

To add a given number of hours to a time, you can add hours divided by 24, or use the TIME function.
In the example shown, the formula in D5 is:
=B5+(C5/24)
Note: make sure results are formatted as time.
How this...

If you want to find the most recent Wednesday, or Saturday, or Monday, given a specific start date, you can use a formula that uses the MOD function.
How the formula works
Formulas that use the concept of "day of week...

To calculate the number of hours between two times, you can use a formula that simply subtracts the start time from the end time. This is useful to calculate working time, calculate elapsed time, etc. However, when...

Excel handles dates and time using a scheme in which dates are serial numbers and times are fractional values. For example, June 1, 2000 12:00 PM is represented in Excel as the number 36678.5, where 36678 is the date...

To add a given number of minutes to a time, you can add minutes divided by 1440, or use the TIME function.
In the example shown, the formula in D5 is:
=B5+(C5/1440)
Note: make sure results are formatted as time....

To generate a fixed value every N columns (for example, a fixed expense every 3 months, a fixed payment every 6 months, etc.) you can use a formula based on the MOD function.
In the example shown, generate a value of...

To highlight numbers that are multiples of a another number with conditional formatting, you can use a simple formula based on the MOD function.
In the example shown, the formula used to highlight multiples of 9 is...

To get the number at a specific place value you can use a formula based on the MOD function. By place value, we mean hundred thousands, ten thousands, thousands, hundreds, tens, ones, etc.
In the example shown, the...

To get the max of every nth column, you can use an array formula based on the MAX, MOD, and COLUMN functions. In the example shown, the formula in M5 is:
=MAX(IF(MOD(COLUMN(B5:K5)-COLUMN(B5)+1,L5)=0,B5:K5))
Note:...

To add an ordinal suffix to a number (i.e. 1st, 2nd, 3rd, etc.) you can use a formula based on the CHOOSE function to assign the suffix.
In the example shown, the formula in C5 is:
=CHOOSE(B5,"st","nd...

To allow only numbers in multiples of 100, you can use data validation with a custom formula based on the MOD function. In the example shown, the data validation applied to C5:C10 is:
=MOD(C5,100)=0
How this formula...

To convert a time from one time zone to another, you can use a formula that converts hours entered as whole numbers to the decimal values that Excel recognizes as times. In the example shown, the formula in F5 is:
=...