Logs are unhelpful at best and thoroughly misleading at worst if not stored with the correct timestamp. A few seconds off is most likely good-enough, but if your log shipping fails for a period of time or you’re trying to make sense of the order of a number of events that happen in quick succession, any inaccuracy is incredibly frustrating.

The unified AWS CloudWatch Agent that replaces the old method of shipping logs and metrics has a facility to read in a timestamp format and then parse your logs against it to extract the exact date & time. If the format doesn’t match the log entry, it’ll be uploaded with the current time, which may be a few seconds or many days wrong. An example timestamp format pattern is shown below:


    "logs": {
        "force_flush_interval": 5,
        "logs_collected": {
            "files": {
                "collect_list": [
                        "file_path": "/var/log/traefik/traefik.log",
                        "log_group_name": "/services/load-balancer/var/log/traefik/traefik.log",
                        "log_stream_name": "{instance_id}",
                        "timestamp_format": "time=\"%Y-%m-%dT%H-%M-%SZ\"",
                        "timezone": "UTC"

The timestamp_format values are specified in the AWS documentation but can be hard to get just right. We can ease the pain slightly by observing that the CloudWatch Agent is converting our JSON configuration into toml:

2019/06/05 17:16:50 I! Detected the host is EC2
Valid Json input schema.
No csm configuration found.
No metric configuration found.
Configuration validation first phase succeeded

2019/06/05 17:16:50 I! Config has been translated into TOML /opt/aws/amazon-cloudwatch-agent/etc/amazon-cloudwatch-agent.toml

If we open up this toml file, we get a parsed version of our timestamp pattern that’s a regular expression:

  file_path = "/var/log/traefik/traefik.log"
  from_beginning = true
  log_group_name = "/services/load-balancer/var/log/traefik/traefik.log"
  log_stream_name = "i-0d4db1f64e660665f"
  pipe = false
  timestamp_layout = "time=\"2006-01-02T15-04-05Z\""
  timestamp_regex = "(time=\"\\d{4}-\\d{2}-\\d{2}T\\d{2}-\\d{2}-\\d{2}Z\")"
  timezone = "UTC"

From there, we can either test out the regex in a tool like rubular or see the layout with an example date to understand where we’ve gone wrong. In my example, I’d put dashes in the time portion of the timestamp rather than colons. Oops!