How the forum member solved it was he added a field in the EmployeeCert Table called PeriodinYears where each certificates validity is added and in the query he added this:
EmployeeCert.PeriodinYears, DateAdd("yyyy",[PeriodInYears],[EffectiveDate]) AS [Expiry Date]
So on the form the text box ExpiryDate now works in relation to each persons date when they attained the certificate and the period of validity for each different certificate.