Cumulative sum when target achieved, get the date

Is your Target always 100 or is it variable?

Essentially I would create a measure to apply a filter show rows where t_cumu is higher than the target and then grab the MIN(Date).

I've hardcoded the target but this could be made dynamic

Target Hit Date = CALCULATE(
					MIN(MyTable[Date]),
					FILTER(
							MyTable,
							MyTable[t_cumu] > 100
						)
					)