My employer is largely a US government/DoD contract operation. The timekeeping to the 15 minute level is mandated by government contracts, so it has become the default for all work whether government or not. PTO can be used in 15 minute increments. Holidays must be 8 hours...not subject to division into smaller chunks. Floating holidays (replacements for Washington's birthday, Lincoln's birthday and one other) are usable in 15 minute increments to a max of 8 hours for a full time employee. The value of a holiday is directly proportional to hours worked. A 40 hour per week employee gets a holiday valued at 8 hours of paid time off. A 20 hour per week employee would get 4 hours of paid time off for that same holiday.
Our pay periods are 2 weeks. If you work 50 hours in week one, you can put 10 hours into a "personal time bank" and submit 40 hours. The 10 hours can be applied to week two on the way to 40 hours. The PTB feature hasn't been of much use to me as most weeks exceed 40 hours so there is no point in using PTB.