Timestamp error when we try to sign a model assertion

[Peng]

I tried to build a image with the steps described in below link:
http://docs.ubuntu.com/core/en/guides/build-device/image-building

In step "2. Sign your model assertion", the command reported the following error:
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error: cannot assemble assertion model: "timestamp" header is not a RFC3339 date: parsing time "$(date -Iseconds --utc)..."
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[Peng (pengliu6)]

The issue can be fixed by replacing
"timestamp": "$(date -Iseconds --utc)" with a valid timestamp. So the document needs to be updated.

[David Callé (davidc3)]

Confirmed. That's interesting since it has been working for me in the past.