A majority of nannies have guaranteed hours, which means that they are paid for a certain amount of hours (IE 36 hours per week) regardless if they work those hours or not. Unfortunately, many employers are guilty of banking hours without realizing that what they are doing is actually illegal. The U.S. Department of Labor goes into great detail about banking hours for a household employee.
What exactly does banking hours mean? Banking hours is when a nanny employer requires the nanny to make up missed hours from the agreed upon schedule. Banking hours means that if a family were to "bank" hours, they would have X amount of hours in the bank to give to the nanny and use until the nanny has received all of the hours.
Example 1: A nanny works Monday-Friday from 3 PM - 7 PM. She is guaranteed 20 hours per week. The family does not need the nanny on Wednesday from until 5 PM, so they have 2 extra hours that they have banked. They ask the nanny to stay late on Friday for 2 hours in order to receive her guaranteed hours.
Example 2: A nanny works Monday-Friday 7 AM - 5 PM. The family does not need the nanny on Tuesday, so they have 10 hours that they have banked. Instead of paying the nanny her guaranteed hours, they begin to sprinkle the "banked" hours throughout the week by having the nanny stay late an hour or two each day in order to receive her guaranteed pay.
In both examples, the nanny is already paid for the guaranteed hours so anything that had been worked outside the normal schedule is not paid extra because the hours have been "banked".
Why you shouldn't do it. Banking hours can effect the relationship between nanny and employer because the nanny will feel disrespected and unappreciated. Yes, it may be frustrating on your end to guarantee 36 hours a week and if you have family visiting and only need the nanny for 30 hours per week you may feel like you are paying for hours you do not use.
Although you cannot bank hours, you can ask the nanny in advance (at least a week in advance) if the nanny would be comfortable coming in an hour earlier or an hour later to make up the 36 hour work week if you only need the nanny for 30 hours. You can ask this with the understanding that the nanny can say no to this request and you would still be responsible for paying the nanny for the 36 hours that were guaranteed. The nanny is only responsible for her agreed upon work schedule (ex: 9-5 PM Monday-Friday).
Out of respect for the nanny and her schedule, you should incorporate guaranteed hours into the contract and if the nanny is not needed for those hours then you should pay her for those hours.