20 CFR §404.1013

1. (a)If part of your work for an employer during a pay period is covered as employment and part excluded, all of your work during that period is considered covered if at least one-half of your time in the pay period is in covered work. If you spend most of your time in a pay period doing work that is excluded, all of your work in that period is excluded.
2. (b)A pay period is the period for which your employer ordinarily pays you. It cannot be more than 31 consecutive days. If the actual period is not always the same, your usual pay period will be used for applying the included-excluded rule.
3. (c)The included-excluded rule does not apply and your covered work will be counted if—
   1. (1)Part of your work is covered by the Railroad Retirement Tax Act and part by the Social Security Act; or
   2. (2)You have no usual pay period of 31 consecutive days or less, or you have separate pay periods for covered and excluded work.