Spark Streaming Guarantee Specific Start Window Time

I'm using Spark Streaming to read data from Kinesis using the Structured Streaming framework, my connection is as follows

val kinesis = spark
  .readStream
  .format("kinesis")
  .option("streams", streamName)
  .option("endpointUrl", endpointUrl)
  .option("initialPositionInStream", "earliest")
  .option("format", "json")
  .schema(<my-schema>)
  .load

The data comes from several IoT devices which have a unique id, I need to aggregate the data by this id and by a tumbling window over the timestamp field, as follows:

val aggregateData = kinesis
    .groupBy($"uid", window($"timestamp", "15 minute", "15 minute"))
    .agg(...)

The problem I'm encountering is that I need to guarantee that every window starts at round times (such as 00:00:00, 00:15:00 and so on), also I need a guarantee that only rows containing full 15-minute long windows are going to be output to my sink, what I'm currently doing is

val query = aggregateData
    .writeStream
      .foreach(postgreSQLWriter)
      .outputMode("update")
      .start()
      .awaitTermination()

Where ths postgreSQLWriter is a StreamWriter I created for inserting each row into a PostgreSQL SGBD. How can I force my windows to be exactly 15-minute long and the start time to be round 15-minute timestamp values for each device unique id?