Add a channel with date/time via JSON read by HTTP Data Advanced sensor

I'm using an HTTP Data Advanced sensor to monitor some JSON, containing several channels. In theory, this JSON should be overwritten every 5 minutes with new, so the sensor runs every 5 minutes too. However, in reality the JSON is not always overwritten.

In order to know when the results in PRTG are from, I'd like to add a channel to the JSON names something like "Query Time (UTC)". How can I successfully add this channel?

Attempt 1 - just throw the date in

However, doing this without any additional parameters in the JSON causes the sensor to fail (not unexpectedly) -

The returned JSON does not match the expected structure ('2018-08-01T10:49:26.0Z' is not a valid integer value). (code: PE231)

{
    "prtg": {
        "result": [
            {
                "channel": "Query Time (UTC)",
                "value": "2018-08-01T10:49:26.0Z"
            },
            {...},
            {...},
            {...}
        ]
    }
}

Attempt 2 - use the 'Custom' unit type

I've also tried to include the "unit" and customUnit parameters, but that leads to another error -

The returned JSON does not match the expected structure ('1 Aug 2018). (code: PE231)

{
    "prtg": {
        "result": [
            {
                "channel": "Query Time (UTC)",
                "value": "1 Aug 2018, 10:49 AM",
                "unit": "Custom",
                "CustomUnit": "UTC"
            },
            {...},
            {...},
            {...}
        ]
    }
}

Attempt 3 - use the 'duration' function

After some research, I came across the 'duration' function. It's not clear from the documentation exactly where it can be used, so I thought I'd give it a go, but that failed too -

The returned JSON does not match the expected structure ('duration(2018-08-01T10:49:26.0000000Z). (code: PE231)

{
    "prtg": {
        "result": [
            {
                "channel": "Query Time (UTC)",
                "value": "duration(2018-08-01T10:49:26.0000000Z, now())"
            },
            {...},
            {...},
            {...}
        ]
    }
}

{
    "prtg": {
        "result": [
            {
                "channel": "Query Age",
                "value": duration('2018-08-01T10:49:26.0000000Z', now())
            },
            {...},
            {...},
            {...}
        ]
    }
}

"channel": "Incoming Messages",
"value": "102