To build a historical timeline slide in PowerPoint with Office Timeline Free or Plus Edition use the historical date feature. It enables you to enter AD dates that precede January, 1, 1753 in the Gregorian calendar. You can enable the historical date feature by right clicking in any date cell in the Milestone or Task Wizard (see below). From there you can enter historical dates back to year January 1, 0001.
If you are using the Plus version for building historical timeline where the year is relevant and the month and day may not be (as shown below), use the Milestone or Task Date Format button on the ribbon and set the date format to YYYY. Also to vertically re-position your Milestones higher and create connectors, so their text does not overlap (4 examples below), please see How To Re-position Crowded Milestones
Browse the free timeline template collection for more examples or free downloadable templates.