android/guava/src/com/google/common/cache/CacheBuilderSpec.java
@NullableDecl
  private static Long durationInNanos(long duration, @NullableDecl TimeUnit unit) {
    return (unit == null) ? null : unit.toNanos(duration);
  }
@NullableDecl
  private static Long durationInNanos(long duration, @NullableDecl TimeUnit unit) {
    return (unit == null) ? null : unit.toNanos(duration);
  }
Converts an expiration duration/unit pair into a single Long for hashing and equality. Uses
nanos to match CacheBuilder implementation.