This is for a report sub-heading and stands on its own. It’s not being used in a sentence. Should all three words be capitalized or just the one?
Since it’s just a report heading, the only purpose is accurately communicating to the report user; I don’t think there is any applicable formal grammar rule.
Personally, I would capitalize all 3 words, to associate it with the abbreviation YTD, which is common in such reports.
Follow the same convention as the other headings.
If you’re using title case for the other headings (capitalizing each word except for the little ones), then use Year-to-Date.
If you’re only using sentence case (capitalizing the first word of the sentence) then use Year-to-date.
The choice would be between two words or one word capitalised: Year-to-Date or Year-to-date. Short prepositions etc, such as “to”, don’t get capitalised in titles.
Compound adjectives also aren’t normally hyphenated except when used attributively. Unless this heading is meant to be interpreted as modifying the title of the section which subsumes it, then you might want to drop the hyphens.