In the "Employment History" section of a contact's record, employments are ordered based on the time period of the employment.  It is important to input the the start date and end date in the time period of an employment to ensure that they display in the right order and also so that they display within the Company record's "Current Employees" and "Former Employees" tabs.  If the time period of employments is not known, there is an option to select a "Current employer" check while editing the employment history.  If the "Current employer" check box is selected, that employment will override the time period dates of all employments.