format string

The "0" custom format specifier serves as a zero-placeholder symbol. If the value that is being formatted has a digit in the position where the zero appears in the format string, that digit is copied to the result string; otherwise, a zero appears in the result string.

You can create a custom numeric format string, which consists of one or more custom numeric specifiers, to define how to format numeric data. A custom numeric format string is any format string that is not a standard numeric format string.
Custom numeric format strings are supported by some overloads of the ToString method of all numeric types. For example, you can supply a numeric format string to the ToString(String) and ToString(String, IFormatProvider) methods of the Int32 type.

Standard numeric format strings are used to format common numeric types. A standard numeric format string takes the form Axx, where A is an alphabetic character called the format specifier, and xx is an optional integer called the precision specifier. The precision specifier ranges from 0 to 99 and affects the number of digits in the result. Any numeric format string that contains more than one alphabetic character, including white space, is interpreted as a custom numeric format string.

Returns a string formatted according to the given format string using the given integer timestamp or the current time if no timestamp is given. In other words, timestamp is optional and defaults to the value of time().