A “preliminary” or “postliminary” activity of the kind referred to in section 4 of the Portal Act is compensable under a contract, custom, or practice within the meaning of that section “only when it is engaged in during the portion of the day with respect to which it is so made compensable.”[84] This provision in no way affects the compensability of activities performed within the workday proper or the computation of hours worked within such workday for purposes of the Fair Labor Standards Act;[85] the provision is applicable only to walking, riding, traveling or other “preliminary” or “postliminary” activities of the kind described in section 4(a) of the Portal Act,[86] which are engaged in outside the workday, during the portions of the day before performance of the first principal activity and after performance of the last principal activity of the employee.[87]