You could write a simple script which would get all item’s started_date using this api end point ‘https://www.wanikani.com/api/v2/assignments’ For more details about this end point go here: API V2 Beta Documentation
This endpoint will provide information for all items you ever learned. So no need for keeping track of items.
EDIT: An example script snippet:
var v2key = 'XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX';
var url = 'https://www.wanikani.com/api/v2/assignments';
var id = '?levels=15';
$.ajax({
url: url + id,
dataType: 'json',
method: 'GET',
headers : { 'Authorization' : 'Bearer token={' + v2key + '}' },
})
.done(function(data) {
console.log(data);
})
.fail(function(err) {
console.log(err);
});