How do I print the "received date" on an invoice?
I came accross the same issue. The invoice date gives the current date and that is in most cases not correct. 

We have used the receive date of the contribution in the invoice template as follows. 

...
{crmAPI var='contribution' entity='Contribution' action='getsingle' id=$id}
...
{ts}Invoice Date:{/ts} {$contribution.receive_date|crmDate:$config-&gt;dateformatFull}
...


The top line with {crmApi...} could be present already in the template. So check whether you can find that line. The {crmApi...} line is not defined before the {$contribution....}