The date of the day I draw the address (which is also the day I write because I don’t draw in advance)
This is also the date registered at postcrossing. If the receiver cannot read/find/register the ID for some reason, having this date will make it easier to find the ID via the help form I think?
I think I did this as well, when I was more active in penpalling. Now, I’ve got a bit lazy and only write the date at the beginning. I wouldn’t repeat the date on every page, though…
For letters, I always put the date at the top right of the page. The current date, not the date it will be mailed or picked up.
For postcards I don’t normally put a date.
I use the date I’m writing, and do the same with letters.
Because, I think it as a log information, “this is written here and then” 
it would feel really weird to me, to write about something that I’m doing or what happened, and then put a date from the past or in the future. Like a fraud 
When I first started postcrossing I would only add the date if someone requested it, just because I didn’t think about it often and I personally didn’t care about it, I didn’t think it was so important. After a while, so many people requested it, I realized maybe it was an expected thing and started adding dates to all postcards, requested or not.
In the beginning I occasionally used the mailing date when I felt like it (always the next day, since mail was picked up around 8am). I almost never talk about “what I did today”, I talk more about hobbies and my cats, so the date doesn’t matter in that context.
Now, I always use date of writing. Sometimes it makes it to the afternoon pick up, sometimes it’s the next day. I also think using the PXing date (always date of writing) might help if there were any issue.
I learned to write the date and city on a letter too, but to me, a postcard is not a letter, so these rules don’t apply.
I am curious why the difference, in terms of putting a date on it (Eliminating the city I can understand – not putting “Emden” on a postcard that says “GREETINGS FROM EMDEN!” on the front of it makes perfect sense…)
If it is a tourist postcard as a sort of souvenir sent from a trip, then I would think the recipient would like to remember when you were on holiday in that place. And if it is an art postcard or something similar that you sent from your own home but used because you didn’t have too long or too sensitive a message and wanted to save on postage, how is that different from it being a letter?
I don’t really know, but I think I see the postcard as something more informal, with less rules to it. Probably because there’s such little space, so you are more free to use it as you wish. But I’ve never really thought about it.
I guess I’m obsessive about it. I make sure that I pick, write and post my cards on the same day. That is, when I request an address, Post crossing will show a date sent, and I make sure I write and send on the same day. Nothing is postmarked on Sundays in the USA, so I never request an address on a Sunday.
Yes and no.
If I remember, I remember. But other than that, like we say in Spain, me da igual ( translates to I don’t care/I don’t give a s***/It doesn’t matter).
Most cards I wrote the day before it gets postmarked.
I always write the postcard the same day I receive the information from the website, therefore I date it that day. To accommodate both American and non-American dating protocols, I’ve compromised by writing “16Apr’21”, for example, to remove all doubt. Verbally it would be said “the sixteenth of April, 2021”.
I only write the date if the profile specifically asks for one
I, too, write the official Postcrossing date on the card, even if I mail it days later, because it will help identify the card if the ID gets damaged in transit.
I don’t have a habit of writing dates on my cards, and I only do so if the Postcrosser requests for it, as it might be something the Postcrosser needs for filing or just for fun.
If I were to write the date, it would be the date I draw the address, as this would be more accurate for Postcrossing help to find the postcard ID should my handwriting not be legible, or the ink smudged on its journey.
I always write place and date of writing on the card, for both may differ from the data given on postcrossing.com and from the cancellation. I often write cards one or two days after pulling an address and put the card in the mailbox next day after collection.
And when I pull addresses on a day trip, maybe the addressee does not like touristy cards, but cards that I have at home, then the place differs from the one on PC.
I usually leave a date on the card and write a date of delivery。
That’s how I write my dates too, although I didn’t consider the US/non-US thing like you did. I too write it 16 April 21, or maybe without the year sometimes…