add invoice date on the invoice - simple way

[♥valerif]

dear all,

 

hope some one can help me

i would like to add an invoice date to my invoices and for cimplicity i thought it is best to use the TEXT_DATE_ORDER_LAST_MODIFIED function

 

however, with my little PHP knowledge i am struggling to implement this feature

 

i wonder if some one could let me know how to create this function

 

i will use this date as my Invoice Date

 

thanks in advance