If specified, only objects with a "last modification time" on or after this timestamp and objects that don't have a "last modification time" are transferred.
last_modified_before fields can be used
together for chunked data processing. For example, consider a script that
processes each day's worth of data at a time. For that you'd set each
of the fields as follows:
last_modified_sinceto the start of the day
last_modified_beforeto the end of the day