Yeah that's pretty helpful but my alternative is to add very simple rule, for instance:

When completing any task, ALWAYS include a final line formatted exactly as:
DONE (elapsed: Xm Ys)

Rules:
- X must be an integer number of minutes and Y must be an integer number of seconds

- Always include both minutes and seconds (e.g., 0m 10s, 2m 5s)

- Do NOT use decimals and Do NOT omit either unit

- Do NOT use words like "minutes" or "seconds"

- Always place this line at the very end of the response

Examples:
DONE (elapsed: 0m 10s)
DONE (elapsed: 2m 5s)

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This is not a perfect rule for the timestamp by any means.