Convert seconds to hhmmss Spark - scala

Question

I have a UDF that creates a timestamp out of 2 field values with date and time. However, the field with time is of seconds format.
So how сan I merge 2 fields of type date and seconds into an hour of a type Unix timestamp?
My current implementation looks like this:
private val unix_epoch = udf[Long, String, String]{ (date, time) =>
deltaDateFormatter.parseDateTime(s"$date $formatted").getSeconds
}
def transform(inputDf: DataFrame): Unit = {
inputDf
.withColumn("event_hour", unix_epoch($"event_date", $"event_time"))
.withColumn("event_ts", from_unixtime($"event_hour").cast(TimestampType))
}
Input data:
event_date,event_time
20170501,87721
20170501,87728
20170501,87721
20170501,87726
Desired output:
event_tmstp, event_hour
2017-05-01 00:22:01,1493598121
2017-05-01 00:22:08,1493598128
2017-05-01 00:22:01,1493598121
2017-05-01 00:22:06,1493598126
Update. data schema:
event_date: string (nullable = true)
event_time: integer (nullable = true)

Answer 1

Cast event_date to a unix timestamp, add the event_time column to get event_hour, and convert back to normal timestamp event_tmstp.
PS I'm not sure why event_time has 86400 seconds (1 day) more. I needed to subtract that to get your expected output.
val df = Seq(
("20170501", 87721),
("20170501", 87728),
("20170501", 87721),
("20170501", 87726)
).toDF("event_date","event_time")
val df2 = df.select(
unix_timestamp(to_date($"event_date", "yyyyMMdd")) + $"event_time" - 86400
).toDF("event_hour").select(
$"event_hour".cast("timestamp").as("event_tmstp"),
$"event_hour"
)
df2.show
+-------------------+----------+
| event_tmstp|event_hour|
+-------------------+----------+
|2017-05-01 00:22:01|1493598121|
|2017-05-01 00:22:08|1493598128|
|2017-05-01 00:22:01|1493598121|
|2017-05-01 00:22:06|1493598126|
+-------------------+----------+

Answer 2

Check below code if this helps without UDF
val df = Seq(
(20170501,87721),
(20170501,87728),
(20170501,87721),
(20170501,87726)
).toDF("date","time")
df
.withColumn("date",
to_date(
unix_timestamp($"date".cast("string"),
"yyyyMMdd"
).cast("timestamp")
)
)
.withColumn(
"event_hour",
unix_timestamp(
concat_ws(
" ",
$"date",
from_unixtime($"time","HH:mm:ss.S")
).cast("timestamp")
)
)
.withColumn(
"event_ts",
from_unixtime($"event_hour")
)
.show(false)
+----------+-----+----------+-------------------+
|date |time |event_hour|event_ts |
+----------+-----+----------+-------------------+
|2017-05-01|87721|1493598121|2017-05-01 00:22:01|
|2017-05-01|87728|1493598128|2017-05-01 00:22:08|
|2017-05-01|87721|1493598121|2017-05-01 00:22:01|
|2017-05-01|87726|1493598126|2017-05-01 00:22:06|
+----------+-----+----------+-------------------+