The scope of what is checked can vary depending on what is needed by the company, position, and customer, and what is blocked by the law. It can be employment history, financial, criminal. It can take a couple of days or it can take months. They might also request non-work references.
When employment checks are done, they don't care about exact dates, but they do care about months. If they discover that the dates were manipulated to hide a hole in employment history or to skip listing a very short they would be concerned. Most companies will only provide employment date, and title information for background checks. They might provide salary information for loan approvals checks.
Financial checks may be required by some employers/positions. They will look into judgements and defaults because that may show lack of judgement. High levels of debt may show a motivation to steal.
Criminal checks are obvious, but what would disqualify a candidate does vary by company and position. Violent crimes are bad for all positions, but reckless driving would be a concern for a position as an ambulance driver.\
advice: If you don't know a date try and determine the date, but then note your uncertainty. Be truthful when answering all questions. The investigator discovering a minor problem that you didn't note on the forms can make the situation worse.