Invoice::setDueDate() must implement interface DateTimeInterface, Xero API using calcinai
<p>I am following this <a href="https://github.com/calcinai/xero-php" rel="noreferrer">wrapper</a></p>

<p>I have this error: <strong>Catchable fatal error: Argument 1 passed to XeroPHP\Models\Accounting\Invoice::setDueDate() must implement interface DateTimeInterface, string given</strong></p>

<p>This is my code:</p>

<pre><code>try{
            $lineitem = new LineItem($this-&gt;_xi);
            $lineitem-&gt;setAccountCode('200')
            -&gt;setQuantity('5.400')
            -&gt;setDescription('this is awesome test')
            -&gt;setUnitAmount('9900.00');

            $contact = new Contact($this-&gt;_xi);
            $contact-&gt;setName("John Doe")
                -&gt;setFirstName("John")
                -&gt;setLastName("Doe")
                -&gt;setEmailAddress("johngwapo@hot.com")
                -&gt;setContactStatus(Contact::CONTACT_STATUS_ACTIVE);



            $invoice = new Invoice($this-&gt;_xi);
            $invoice-&gt;setType(Invoice::INVOICE_TYPE_ACCREC)
                -&gt;setStatus(Invoice::INVOICE_STATUS_AUTHORISED)
                -&gt;setContact($contact)
                //-&gt;setDate(\DateTimeInterface::format("Y-m-d"))
                -&gt;setDueDate("2018-09-09")
                -&gt;setLineAmountType(Invoice::LINEAMOUNT_TYPE_EXCLUSIVE)
                -&gt;addLineItem($lineitem)
                -&gt;setInvoiceNumber('10')
                -&gt;save();



        }catch ( Exception $e ){
            $GLOBALS['log']-&gt;fatal('[Xero-createContact]-' . $e-&gt;getMessage());
            echo $e-&gt;getMessage();

        }
</code></pre>

<p>When I tried doing it like this:</p>

<pre><code>-&gt;setDueDate(\DateTimeInterface::format("Y-m-d"))
</code></pre>

<p>I got this error instead: <strong>Fatal error: Non-static method DateTimeInterface::format() cannot be called statically, assuming $this from incompatible context</strong></p>

<p>This is the function of setDueDate that I am calling:</p>

<pre><code> /**
     * @param \DateTimeInterface $value
     * @return Invoice
     */
public function setDueDate(\DateTimeInterface $value)
    {
        $this-&gt;propertyUpdated('DueDate', $value);
        $this-&gt;_data['DueDate'] = $value;
        return $this;
    }
</code></pre>

<p>I'm really lost here as to how do I use this DateTimeInterface and how can I even set a future date using it, and how do I solve all this errors.</p>