Petro did not attend the 7 August ceremony in Cali, which he had publicly opposed; he is absent from the inauguration's attendance records.
The Petro attendance market resolves on his physical presence at the inauguration ceremony, per official ceremony records and the live state broadcast.
In every close runoff we found, the winner took office on time. We looked for Latin American presidential runoffs since 1994 that were won by a very small margin, and found four. Whatever the losing side claimed, the declared winner took office on the date set by the constitution. One of the four, Mexico in 2006, was a single round of voting rather than a runoff.