Hi Richard,
Unfortunately it's not possible to filter by, or determine, when a given keyword was added. The best approximation is to look for keywords that had greater than one impression during a given time period:
var keywordsIterator = AdWordsApp.keywords()
.forDateRange('20120101', '20120201')
.withCondition('Impressions > 0')
.get();
Best,
- Eric Koleda, AdWords scripts team