awaitility/src/main/java/org/awaitility/Duration.java
public Duration plus(long amount, TimeUnit timeUnit) {
        if (timeUnit == null) {
            throw new IllegalArgumentException("Time unit cannot be null");
        }

        return new Plus().apply(this, new Duration(amount, timeUnit));
    }
public Duration plus(long amount, TimeUnit timeUnit) {
        if (timeUnit == null) {
            throw new IllegalArgumentException("Time unit cannot be null");
        }

        return new Plus().apply(this, new Duration(amount, timeUnit));
    }
Add a amount with the given time unit. For example:
<pre>
new Duration(2, SECONDS).plus(4, MILLISECONDS)
</pre>
will return a Duration of 2004 {@link TimeUnit#MILLISECONDS}.
<p>
<pre>
new Duration(2, SECONDS).plus(1, MINUTES)
</pre>
will return a Duration of 62 {@link TimeUnit#SECONDS}.

@param amount   The amount to add
@param timeUnit The time unit to add
@return A new amount