The terms include three references to dates. If your receipt does not show the date of the visit, how does Skyguide verify that you (a) have completed the visit, (b) are claiming for only one visit in a month and (c) that you have mailed the claim within 30 days of the visit?
They have never asked for proof of visit date, only that the receipt be mailed within 30 days of use. I have always sent them my receipt, which is dated the day I purchased the pass (in the past from delta.com or aa.com and more recently from ebay). Along with a completed form which asks for the date the pass was used. If they want a receipt showing date used, not purchased, then the terms should clearly state that. Unfortunately that would be difficult to produce. I know AA has in the past written "used" and a date by hand with a pen on the passes. They don't give you another receipt. The only way I know to actually get a receipt for date of use would be to purchase the pass at the desk, which I do not like doing as I've had lounges turn me away if they are restricting access. If you already have a day pass, they usually accept it and allow entry. They are less likely to turn you away if you've already spent the money on the pass vs. you are standing there asking to buy one.