The Time Harvester was a device supplied by Mr Symington and Mr Blenkinsop as help for business people. Both co-workers, Andrew Brown and Sameera Jenkins were competing against each other for a promotion, using these Time Harvesters as an advantage to impress important people. The interest could be paid back either all at once or just 10% at a time.
Amy was persuaded by Sameera to have one. She found it so addictive, due to the extra time it provided and the feeling of life "melting" into her, that she crossed over her timeline many times, to help Rory and then the Eleventh Doctor and also meet her parents within twenty-four hours. The Doctor was displeased. He tried to remove it from her arm, but it was held sealed. Amy soon discovered that the interest, as it acted as compound interest rather literal, on the Time Harvesters was much more than she believed. Using his Sonic screwdriver, the Doctor could make it display: "BORROWED TIME TOTAL SINCE LAST REPAYMENT: 9 DAYS, 1 HOUR. INTEREST TERMS: 5 MINUTES PER HOUR, PER HOUR. TOTAL TIME OWED: 55,000 YEARS."
Mr Symington and Mr Blenkinsop arrived, telling her that it was against the law to remove the watches and would mean that her life would end completely. The Doctor, due to his Time Lord biology, was able to pay off the hundred years of debt without physically feeling it beyond a wince. (PROSE: Borrowed Time)