public class TimeTools {

    private final static long SECONDS_IN_DAY = 3600*24;
    private final static String DATE_FORMAT = "dd.MM.yyyy";

    public static long dateToDayNumber(Date date) {
        DateFormat df = new SimpleDateFormat(DATE_FORMAT);
        long msLocal = 0;
        try {
            msLocal = date.getTime() - df.parse("01.01.1970").getTime();
        } catch (ParseException e) {
            e.printStackTrace();
        }
        return msLocal / (1000*SECONDS_IN_DAY);
    }

    public static Date dayNumberToDate(long day) {
        DateFormat df = new SimpleDateFormat(DATE_FORMAT);
        long msLocal = day*1000*SECONDS_IN_DAY;
        long msZero = 0;
        try {
            msZero = df.parse("01.01.1970").getTime();
        } catch (ParseException e) {
            e.printStackTrace();
        }
        return new Date(msLocal + msZero);
    }

    public static void main(String[] args) {
        Date someDate = DateString.convert("02.01.2012 00:00", "dd.MM.yyyy HH:mm");
        long dayNumber = dateToDayNumber(someDate);
        System.out.println("day's number: " + dayNumber);
        System.out.println("date is: " + dayNumberToDate(dayNumber));
    }

}

day's number: 15340
date is: Sun Jan 01 01:00:00 MSK 2012

        DateFormat df = new SimpleDateFormat(DATE_FORMAT);
        Date someDate = null;
        try {
            someDate = df.parse("02.01.2012");
        } catch (ParseException e) {
            e.printStackTrace();
        }