Is unused sick leave creditable under FERS?
Yes, but it’s not a day-to-day relationship. For this purpose, the Office of Personnel Management treats all months as having 30 days. It divides 360 into the congressionally mandated number of work hours in a year, which is 2087. The net result is that 5.797+ hours of sick leave equals one day. See the table in

Credit for Unused Sick Leave in Chapter 3, Section 3 of the Federal Employees Almanac.
Unused sick leave cannot be used in the computation of a high-3 salary or for meeting the minimum length of service for retirement eligibility. It also is not creditable toward a deferred annuity.

Note: For retirements under FERS before calendar year 2014, only partial credit or no credit generally was given. See that section of the Federal Employees Almanac for details.