The best practice for this, in my opinion is:
TimeUnit.DAYS.toMillis(1); // 1 day to milliseconds.
TimeUnit.MINUTES.toMillis(23); // 23 minutes to milliseconds.
TimeUnit.HOURS.toMillis(4); // 4 hours to milliseconds.
TimeUnit.SECONDS.toMillis(96); // 96 seconds to milliseconds.