VALLADOLID, Spain—Even though she's one of millions of young, unemployed Spaniards, 22-year-old Silvia Martín takes comfort in knowing that her bank is still standing behind her. It's not a lending institution, but rather a time bank whose nearly 400 members barter their services by the hour.

Ms. Martín, who doesn't own a car and can't afford taxis, has relied on other time-bank members to give her lifts around town for her odd jobs and errands, as well as to help with house repairs. In return, she has cared for members' elderly...