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If the event is on the first day of the month and no time is specified it shows only the month. The same is true for year. If the event date is the first day of the first month of the year, it only shows the year.
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Simply enter the year. Earlier versions of TimelineJS required you to enter a date as January 1 of a given year with no time to do this, but now you should just enter the full year (four digits for recent events, or negative numbers for BC) in the "start date" or "end date" columns. You may need to change the format of the cells in Google Spreadsheet so that it doesn't change your plain year into a full date. To do this, select all the cells you want to change and select "Format > Number > Plain Text" from the Google Spreadsheet menus.