The decimal value is used in formatting fractions of seconds
public function setDecimal( string $decimal, ): void;
If the skeleton contains fractional seconds, then this is used with the fractional seconds. For example, suppose that the input pattern is "hhmmssSSSS", and the best matching pattern internally is "H:mm:ss", and the decimal string is ",". Then the resulting pattern is modified to be "H:mm:ss,SSSS"
string $decimal- The string to represent the decimal