Calculate Probation period (+2 month)

Hi everyone,

I am working on calculate probation period (2 month) since the start date, I used formula =DATE(YEAR(Startdate@row); MONTH(Startdate@row) + 2; DAY(Startdate@row))

The month was fine except November and December (calculate more but the time did not jump to next year).

Please give me advise for this, thank you so much