Sql Server Self JOIN (pushing column values down)

Sql Server Self JOIN (pushing column values down)

I am asked to do the following: "CycleStartDate needs to be the BillDate
from the previous BillDate record. If a previous record does not exist,
you should use the most recent CycleEndDate from the DataTime table"
CycleStartDate and CycleEndDate are columns in a table called DataTime
BillDate is a column in a table called BillingData
This is the BillDate values:
2012-07-27 00:00:00.000
2012-07-27 00:00:00.000
2012-08-27 00:00:00.000
2012-08-27 00:00:00.000
2012-09-28 00:00:00.000
2012-09-28 00:00:00.000
2012-10-26 00:00:00.000
2012-10-26 00:00:00.000
2012-11-27 00:00:00.000
2012-11-27 00:00:00.000
2012-12-27 00:00:00.000
How would I set the CycleStartDate values based on the requirements? The
tables Datetime and BillingData are connected by a column called MeterID.