/** This should always be called before attempting to call sendPayment. */
public boolean isExpired() {
return paymentDetails.hasExpires()
&& System.currentTimeMillis() / 1000L > paymentDetails.getExpires();
}
/** Returns the expires time of the payment request, or null if none. */
@Nullable
public Date getExpires() {
if (paymentDetails.hasExpires()) return new Date(paymentDetails.getExpires() * 1000);
else return null;
}