pyspark 2.4 How to Format nested field to datetime from epoch

Question

After reading in Parquet Files using pyspark 2.4 I would like to format a field within a struct and array. I would like to replace the doctime field that is in epoch time(e.g. 1280620800000) to a formatted datetime. How would I construct a withcolumn and how to format epoch to datetime?

root
 |-- Key: struct
 |    |-- subkey: array
 |    |    |-- subkey1: struct
 |    |    |    |-- name: string
 |    |    |    |-- val: struct
 |    |    |    |    |-- text: string
 |-- Data: struct
 |    |-- subdata: array
 |    |    |-- subdata1: struct
 |    |    |    |-- key1: long
 |    |    |    |-- value: struct
 |    |    |    |    |-- numeric: double
 |    |    |    |    |-- string: string
 |    |    |    |-- doctime: long
 |    |    |    |-- tag: integer
 |    |    |    |-- rptime: long

Any help is greatly appreciated.