Is there a formal ontology for documenting historical timelines?

Is there a common process/system/platform that researchers use to document historical timelines?

There is no standard of broad use by historians. By nature, the format and quality of time-based data is different in each domain. This makes thing difficult for a platform to emerge or a standard for people to agree to. Yet, I founded a start-up to change just that, have a look at HistoryTimeline.com. We work to create a Timeline Software that helps professional historians browse and organize the time space, like Google Maps does for geography.

how do historical researchers organize their notes?

Researchers typically use the standard office suite of programs: mostly text editor, sheets, etc. Sometimes they use databases and any other advanced tech available, for instance, in very well-funded projects. The problem is, historians transmit their information by telling stories, this means books, often lengthy books. This is because the historical tradition since Ancient Greece and beyond (we humans like stories), but also because the linguistic turn, which had quite an impact in early 20th century historians. Yet this is now a less dominant approach, thanks to modern data-based efforts to renew history research.

[Music Notation] Does anything like this exist for historical timelines?

Yes, Timeline Software (please see the disclaimer below). Is not a coincidence that you mention that. Organizing and documenting timelines requires knowledge in time notation, and music offers plenty.

storied history of the Gucci family and company

Storied indeed. This Gucci company timeline you mention reminds me of this Microsoft History Timeline I published during mid 2021. Using Timeline Software you can organize and visually track events, people, periods, locations people spend time on, etc Like you ask.

You certainly can use the software in your own projects using any type of data.

1st Disclaimer: I have a university degree in Musical Composition (ESMUC, 'Superior' School of Music of Barcelona), my musical thesis is about Time Notation types and techniques. I also have a university degree in History (Univesity Barcelona), and my history thesis is about Roman Calendars & Chronology. Both theses are unpublished, if interested, I can provide access.

2nd Disclaimer: This answer contains personal references to my work as a professional. I've tryed to stay on topic. I'll edit anything needed to adapt to the guidelines of this forum.

Historiographical scholarship is not based on computing models of information. Historiography is a story. Historiographical scholarship is predominantly a multiform narrative containing smaller narratives: historiography is a literary endeavor. This is because historians are concerned with discovering and conveying systems of human meaning, and, because humans are self-narrativising. Humans are basically walking stories about themselves. While other examples of conveyance of knowledge of the past exist, such as tabular data, poetic moments, or occasionally history which strays from the long form narrative towards drama, it is narrative which drives historical organisation of knowledge and wisdom.

In a hierarchy of ideas I've seen replayed often: data -> information -> knowledge -> wisdom. Historians usually interact with already highly structured knowledge. The evil man wrote a diary before he died. The good organisation minuted its meetings. The indifferent secretary filed all the notes, no all the notes, into topical files. The monstrously insentient system of relations of production created knowledge repositories to organise market behaviour which daily reported on all things from numerous ideological slants for public consumption and better stock purchasing. Historians rarely if ever encounter data or information: they deal with past knowledges.

Historians rarely if ever organise past knowledges. Past knowledge is organised by a highly skilled set of information professionals known as the GLAM sector: Gallaries, Libraries, Archives and Museaums. For the historian, the action of Librarians in organising public knowledge as it is produced and of Archivists in organising private knowledge as it is seized or abandoned means that historians have the majority of their content pre-organised for them. In general they will read journals, chapters and books which were public; and then read known topical archives or general archives searching for topics which were private. As the historian reads that historian already contains a key narrative which they have preproduced from reading publicly available narratives (other histories). Their narrative may have a different theme or focus character (for example, instead of the Australian Labor Party, it may be the NSW Right faction who is their lead character). They then incorporate themes incidents topics or characters into their narrative from the archival material, usually by archivally organised notes:

2022-03-20 Noel Butlin Archive Canberra (ACT, AU) [archive retrieval data, such as its reference number] South Sydney Branch files, Minutes books, Minute book 1944 at 13 April 1944: Previous Minutes: Fred has really gone the hack on John here over bad minuting in the last meeting, Fred's got it in for John, read for this in future, maybe backtrack if they do hate each other.

As a historian prepares chapters for publication, they'll reference their minutes which are generally organised in the order of their reading, with sufficient citeable references that other historians will believe their reading, and they tie these points in. But their internal narrative tends to be overriding: before writing this chapter they knew what the story was from all their reading of everyone else's stories: their mind has internally organised the story, and they then slot the evidentiary material into their organised story.

Humans are story machines, history is a narrative member of the humanities which works on stories. Humans internally organise stories to make sense of the world, and historians here are no different. What is different is what stories historians have read before they make up a story and go read stories that were made up by people really close to the story that they made up in private. Historians stick a big lever into public/private consumption stories and open up the cracks. We read your diaries. We read your organisation's minutes. We read the secret diary you kept of how your organisation's minutes were wrong. We read the hidden stories you didn't even know you were writing as you were writing them via your word choice which reveals: the things you sought to conceal, or which you concealed even from yourself. We read your stories into our story and then write that down.

The information management side is trivial, you just copy the citation information off the librarian or the archivist (or rarely the gallery or museum curator) and stick it up top, possibly in a field, possibly just for deep text search.

Annotated Bibliography

Hayden White (1996) Interview. interloq. JE Paz Soldán Lucero 6 ( https://escholarship.org/content/qt5r30d183/qt5r30d183_noSplash_084f02930355392feab84b6918b978d6.pdf?t=pe4m67 ) : chiefly on facts and facticity, and the constructed nature of claims in language; secondarily on humanities' claims of exceptionalism.