i18n\Formatter $datetimeFormat

The default format string to be used to format a date and time. This can be "short", "medium", "long", or "full", which represents a preset format of different lengths.

It can also be a custom format as specified in the ICU manual.

Alternatively this can be a string prefixed with php: representing a format that can be recognized by the PHP date()-function.

For example:

'MM/dd/yyyy HH:mm:ss' // date and time in ICU format
'php:m/d/Y H:i:s' // the same date and time in PHP format