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I recommend to display both “post Published date” and “Last Modified date”.
The case of the display of only the “Post Published Date”, visitors will stop reading because posts have written old date.
The case of the display of only the “Last Modified Date”, visitors do not know when that article was written.
Display the Post Published Date
<?php the_time('Y/m/d'); ?>
Display the Last Modified Date
<?php the_modified_date('Y/m/d') ?>
more detailed setting
If you want a more detailed settings, please refer to the link below.
Formatting Date and Time-WordPress Codex