logstash timestamp on year rollover

We use logstash to store/search logs from our mail servers. I noticed today that we didn't have any indices from this year (2015). Quick investigation showed that current logs were being stored as 2014.01.05 (ie same day but last year) and these indices were being deleted by a cron job that looks for old indices.

Restarting logstash fixed things so I assume that logstash is filling in the year information based on the time it started.

We're running Logstash 1.4.1 with Elasticsearch 1.2.4. So not the latest version of Elasticsearch but I don't see anything relevant in the changelog for 1.4.2.

Log entries are sent to logstash using syslog - config below along with example of input line and parsed output.

Is there a better fix for this than just remembering to restart Logstash on New Year's day?

Example of input line

Jan  5 15:03:35 cheviot22 exim[15034]: 1Y89Bv-0003uU-DD <= [email protected] H=adudeviis.ncl.ac.uk (campus) [10.8.232.56] P=esmtp S=2548 [email protected]

{
  "_index": "logstash-2014.01.05",
  "_type": "mails",
  "_id": "HO0TQs66SA-1QkQBYd9Jag",
  "_score": null,
  "_source": {
    "@version": "1",
    "@timestamp": "2014-01-05T15:03:35.000Z",
    "type": "mails",
    "priority": 22,
    "timestamp": "Jan  5 15:03:35",
    "logsource": "cheviot22",
    "program": "exim",
    "pid": "15034",
    "severity": 6,
    "facility": 2,
    "facility_label": "mail",
    "severity_label": "Informational",
    "msg": "1Y89Bv-0003uU-DD <= [email protected] H=adudeviis.ncl.ac.uk (campus) [10.8.232.56] P=esmtp S=2548 [email protected]",
    "tags": [
      "grokked",
      "exim_grokked",
      "dated"
    ],
    "xid": "1Y89Bv-0003uU",
    "exim_rcpt_kv": "[email protected] H=adudeviis.ncl.ac.uk (campus) [10.8.232.56] P=esmtp S=2548 [email protected]",
    "H": "adudeviis.ncl.ac.uk",
    "P": "esmtp",
    "S": "2548",
    "id": "[email protected]"
  },
  "sort": [
    1388934215000,
    1388934215000
  ]
}

Logstash config (with irrelevant bits removed) ...

input {
    syslog {
        codec => "plain"
        debug => false
        port => 514
        type => "mails"
    }
}

filter {
    mutate {
        remove_field => [ "path", "host" ]
    }

    if [type] == "mails" {
        grok {
            patterns_dir => [ "/etc/logstash/patterns" ]
            match => [ "message",  "(?<msg>.*)" ]
            add_tag => [ "grokked" ]
            break_on_match => true
            remove_field => [ "message" ]
        }
    }

    date {
        match => [ "timestamp", "ISO8601", "MMM dd HH:mm:ss", "MMM  d HH:mm:ss"]
        add_tag => [ "dated" ]
    }
}

output {
        elasticsearch {
                cluster => "logstash"
        host => "iss-logstash01"
        flush_size => 1000
        index => "logstash-%{+YYYY.MM.dd}"
        }
}