strftime, the popular function for formatting date-time (with PHP, Ruby, Python and C) has many parameters which are usually hard to remember.
There are several web-based references that ease the usage of
strftime by enabling us to view the details of each parameter and test them instantly. Here they are:
Strfti.me simply lists the parameters for dates + times separately with an explanation to each and allows us to view the output of any custom format.
The website displays all the parameters where each of them can be clicked to create the custom function.
P.S. It would be much better to see the result of the function instantly (feature request).
Besides presenting the parameters, there are presets for for the widely-used formats.
Also, there is a drag 'n' drop custom format builder.