elasticsearch date arithmetic behaviour oddity

appears to cache results from first execution

{
    "sort": [
        {
            "publication_time": {
                "order": "desc"
            }
        }
    ], 
    "from": 0, 
    "size": 3,
    "fields": [
        "headline"
    ], 
    "query": {
        "query_string": {
            "query": "+publication_time:[now-5m TO now]"
        }
    } 
}

works as expected

{
    "sort": [
        {
            "publication_time": {
                "order": "desc"
            }
        }
    ], 
    "from": 0, 
    "size": 3,
    "fields": [
        "headline"
    ], 
    "query": {
        "query_string": {
            "query": "+publication_time:[now-5m TO 2013-03-23]"
        }
    } 
}