いちおう動くようにはなった。

CalcWorkingTimeWithNewAPI.java

import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.UnsupportedEncodingException;
import java.time.Duration;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.logging.Level;
import java.util.logging.Logger;

public class CalcWorkingTimeWithNewAPI {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        
        String user = "hoge";
        String line;
        
        DateTimeFormatter dtf1 = DateTimeFormatter.ofPattern("yyyyMMdd");
        DateTimeFormatter dtf2 = DateTimeFormatter.ofPattern("yyyy/MM/dd");
        DateTimeFormatter csvFormat = DateTimeFormatter.ofPattern("yyyy/MM/dd []H:mm:ss");
        DateTimeFormatter csvFormat2 = DateTimeFormatter.ofPattern("yyyy/MM/dd []H:[]m");
        DateTimeFormatter hour = DateTimeFormatter.ofPattern("HH");
        DateTimeFormatter min = DateTimeFormatter.ofPattern("mm");
        
        String today = dtf1.format(LocalDateTime.now());
        String filename = "C:\\Users\\hoge\\Desktop\\" + today + ".csv";
        
        try {
            //ファイルを取得して文字コード調整
            BufferedReader br = new BufferedReader(new InputStreamReader(new FileInputStream(filename), "SJIS"));
            
            try {
                //読み込み開始
                while ((line = br.readLine()) != null) {
                    String[] csvarr = line.split(",");
                    
                    //自分の行かつ出退勤が両方記載されている行のみ対象とする
                    if(csvarr[0].equals(user) && csvarr.length == 3){
                        
                        String shukkin = csvarr[1];
                        String taikin = csvarr[2];
                        
                        //LocalDateTimeに変換する
                        LocalDateTime ldt1 = LocalDateTime.parse(shukkin, csvFormat);
                        LocalDateTime ldt2 = LocalDateTime.parse(taikin, csvFormat);
                        
                        //日付・時・分それぞれに分割
                        int shukkinhour = Integer.parseInt(hour.format(ldt1));
                        int taikinhour = Integer.parseInt(hour.format(ldt2));
                        int shukkinmin = Integer.parseInt(min.format(ldt1));
                        int taikinmin = Integer.parseInt(min.format(ldt2));
                        String Strworkday = dtf2.format(ldt1);
                        
                        //15分刻みで計算 45分以降出勤の場合出勤のMMを1繰り上げる
                        switch(shukkinmin / 15){
                            case 0:
                                shukkinmin = 15;
                                break;
                            case 1:
                                shukkinmin = 30;
                                break;
                            case 2:
                                shukkinmin = 45;
                                break;
                            case 3:
                                shukkinmin = 0;
                                shukkinhour ++;
                                break;
                        }
                        switch(taikinmin / 15){
                            case 0:
                                taikinmin = 0;
                                break;
                            case 1:
                                taikinmin = 15;
                                break;
                            case 2:
                                taikinmin = 30;
                                break;
                            case 3:
                                taikinmin = 45;
                                break;
                        }
                        
                        //diffを求めるため再度LocalDateTimeに変換
                        String time1 = Strworkday+" "+shukkinhour+":"+shukkinmin;
                        String time2 = Strworkday+" "+taikinhour+":"+taikinmin;
                        LocalDateTime shukkintime = LocalDateTime.parse(time1, csvFormat2);
                        LocalDateTime taikintime = LocalDateTime.parse(time2, csvFormat2);

                        //差分を計算
                        Duration difftime = Duration.between(shukkintime, taikintime);
                        int work = (int)difftime.toMinutes();
                        
                        //60で割った商とあまりのMM変換をdoubleでたす
                        int workhour = work/60;
                        double workmin = 0;

                        switch( work % 60 ){
                            case 0:
                                workmin += 0;
                                break;
                            case 15:
                                workmin += 0.25;
                                break;
                            case 30:
                                workmin += 0.5;
                                break;
                            case 45:
                                workmin += 0.75;
                                break;
                        }
                        
                        //休憩時間をマイナス
                        double worktime = workhour -1 + workmin;
                        
                        //出力
                        System.out.println(Strworkday+": "+worktime);
                    }
                }
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        } catch (UnsupportedEncodingException | FileNotFoundException ex) {
            System.out.println("csvの文字コードがおかしいか、ファイルがないのでよみこめません");
        }
    }
}