Implement endpoint to retrieve last modified GK

[kumy]

Raised in geokrety/geokrety-scripts#8

We should provide endpoints to permit retrieve updated/created GK since a date-time, it will help partners to synchronize from this source instead of export*.php scripts which are quite cpu intensive.

[kumy]

It should mimic current behavior of export*.php ie: ?modifiedsince=20100901000000 See doc https://new-theme.staging.geokrety.org/api.php

[kumy]

Ho seems it's already there ! 🤣

gkm-api-php/gkm/select.php

Lines 29 to 33 in f205ad3

	// since modifiedsince
	} else if (isset($_GET['modifiedsince'])) {
	$query = query($session, 'select-by-date.xq');
	$query->bind('modifiedsince', strtoupper($_GET['modifiedsince']), 'xs:dateTime');

[kumy]

However, as the database includes only date (not datetime), it's not enough to be closed.