This is my husband's situation exactly. Over 20 years without a change in employer, but his division was bought out, he's changed locations and job descriptions multiple times as he's moved on in his career.
This is how he handles it: On the resume, put a header reflecting your job title, followed by the employer, then dates (as best you can remember). Then he describes his main accomplishments in that position. Jobs are listed in order from most recent back. If you have a long work history, the earlier jobs can be a one-liner.
As an example:
"CHIEF NURSING OFFICER, ABC Hospital, Inc. (formerly XYZ Hospitals), Santa Monica Headquarters, 2003 to present. Oversaw all leadership and directive aspects of nursing care in 50,000 bed hospital with operating budget of $2 billion annually. All JCAHO inspections passed during my tenure.
FLOOR NURSING SUPERVISOR, XYZ Hospitals (now known as ABC Hospital, Inc.), San Bernardino Hospital, 2001 to 2003. Supervised nursing and support staff on medical-surgical floor with 80 beds. Patient satisfaction scores and quality scores consistently in the highest 2% of XYZ Hospitals system.
FLOOR NURSE, XYZ Hospitals (now known as ABC Hospital, Inc.), Simi Valley Community Health Services, 1997 to 2001. Performed patient care, staff training, and supervision of support staff for medical-surgical patients."
About the dates, we usually use years only rather than getting too much into the detail. Frankly, if you don't remember it, no one else will either.
Try to show your progress from job to job. This will avoid the trap of looking like a job-hopper if you have a real reason for changing jobs.
I hope this helps you. Excuse the exaggeration for effect.