Whether you are putting together a resume, filling out an application or just keeping track for your personal records, at some point you will need to list your work history and the period of time of your employment at each post. There are a couple of questions to ask in advance of your calculation regarding how to best figure how long you have worked somewhere and how to list it. For instance, if you have some period of months where you were between jobs, you may want to list only years of employment and leave out the months. To show consistent employment, list more detail.
Determine the starting date of the job in question and use that month as the first month of employment to calculate your employment showing the months. It is fine if the month overlaps with time spent on a prior job, as that will help show continuous employment.
Use the year you began the job if you are not intending to break down your employment history by the months. Again, an overlap with another job in the same year is fine.
Determine the closing date for the job using the method you have chosen (month or year).
List the dates according to the method you choose and be consistent. For example, if you are listing the months for your jobs, you could show them in text (September 2009-March 2010) or numerical format (9/2009-3/2010), but use the same format for all the jobs. If you began your next job in March 2010 and still have it, list it as "March 2010-present" or "3/2010-present." For listing using only the years you would list the two jobs as "2009-2010" and "2010-present."
Calculate the time period you have worked at a particular job from the record of the job history you have created, 12 months for each full year and adding the months of partial years from the beginning and the end of your tenure to the total.