An Easy Mnemonic to Format Dates As Strings in C#

30 Mar 2025 #csharp

Is it MM or mm for months when formatting dates? Is it yyyy-MM-dd or yyyy-mm-dd?

I always forgot which one to use…until I figured out an easy mnemonic.

MM vs mm

Ask yourself: Which one represents a larger time frame? Months or minutes?

Since one month is larger than one minute, it’s M for months. An uppercase m is larger than a lowercase m. Write, anyDate.ToString("yyyy-MM-dd").

Then, what is it for milliseconds? Isn’t it mm?

Now, ask yourself: Which one is faster? One minute or one millisecond?

Since one millisecond is faster, write f instead of m. Write anyDate.ToString("hh:mm:ss.fff").

Easy, peasy now!