Utility functions that can be called in Salesforce Apex flows.

FlowUtils.cls

public class FlowUtils {
    @InvocableMethod(label='Format Date/Time')
    public static String[] format(FormatInput[] inputs) {
        String[] outputs = new String[0];
        for (FormatInput input : inputs) {
            outputs.add(input.dt.format(input.format, input.tz));
        }
        return outputs;
    }

    public class FormatInput {
        @InvocableVariable(
            label='DateTime'
            description='DateTime to format.'
            required=true
        )
        public DateTime dt;

        @InvocableVariable(
            label='Format'
            description='How to format the DateTime.  Format details can be found at http://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html.'
            required=true
        )
        public String format;

        @InvocableVariable(
            label='Time Zone'
            description='The time zone used to format the DateTime (eg. "America/New_York").'
            required=true
        )
        public String tz;
    }
}