public DateTime minus(DateTime dt, int n) {
		DateTime start = dt;
		for (int i = 0; i < n; i++) {
			start = start.minus(unitPeriod);
		}
		return start;
	}
Subtracts this resolution a given number of times from a supplied date.

@param dt the date to subtract from
@param n the number of periods of this resolution to subtract
@return the resulting date in the past.
spring-xd-analytics/src/main/java/org/springframework/xd/analytics/metrics/core/AggregateCountResolution.java