Integer division is suspicious when the target type of the expression is a float. For example:

private static final double ONE_HALF = 1 / 2; // Actually 0.

If you specifically want the integer result, consider pulling out variable to make it clear what's happening. For example, instead of:

// Deduct 10% from the grade for every week the assignment is late:
float adjustedGrade = grade - (days / 7) * .1;

Prefer:

// Deduct 10% from the grade for every week the assignment is late:
int fullWeeks = days / 7;
float adjustedGrade = grade - fullWeeks * .1;

Similarly, multiplication of two int values which are then cast to a long is problematic, as the int multiplication could overflow. It's better to perform the multiplication using long arithmetic.

long secondsToNanos(int seconds) {
  return seconds * 1_000_000_000; // Oops; starts overflowing around 2.15 seconds.
}

Instead, prefer:

long secondsToNanos(int seconds) {
  return seconds * 1_000_000_000L; // Or ((long) seconds) * 1_000_000_000.
}