You can calculate the date N years prior to a specified date, with one year being equal to 365 days.
In kintone, values in Date, Time, and Date and time fields are treated as UNIX time. This means that dates and times are treated in seconds, so you need to use seconds as the unit of time in your formula. For example, enter "60" to indicate one minute. Enter "3600" or "60*60" to indicate one hour, and enter "86400" or "60*60*24" to indicate one day.

• If you want to calculate the date two years prior to the date entered in a "Date" field
  Place a "Calculated" field on the app form and set the following formula.

  • Formula: Date-(2*365*24*60*60)
  • Format: Date (Aug 06, 2012)

  Note that this sample formula does not take into account leap years (366 days in a year).