There is a rather (read very) clunky way to update the timeline if you need this for a one-off upload. Assuming you have a board with line-items already populated (potentially including separate start and end date columns) and an Item ID column , you can download the board to Excel. Using formulas you can then create a standard Mutate API command (see example below) insert the dates for the timeline, copy-paste the values into the API Playground, hit prettify and then execute.
mutation {
Update1: change_multiple_column_values(
item_id: xxxxxxxxxx
board_id: xxxxxxxxxx
column_values: “{"project_timeline" : {"from" : "2023-03-28", "to" : "2023-05-30"}}”
) {
id
}
Update2: change_multiple_column_values(
item_id: xxxxxxxxxx
board_id: xxxxxxxxxx
column_values: “{"project_timeline" : {"from" : "2023-09-15", "to" : "2023-12-12"}}”
) {
id
}
}