I know Flexviewer is "old", but we are still using it.
I am aware of the UTC setting:
<SPAN class="token tag"><SPAN class="token tag"><SPAN class="punctuation token"><</SPAN>format</SPAN> <SPAN class="attr-name token">dateformat</SPAN><SPAN class="attr-value token"><SPAN class="punctuation token">=</SPAN><SPAN class="punctuation token">"</SPAN>shortDate<SPAN class="punctuation token">"</SPAN></SPAN> <SPAN class="attr-name token">useutc</SPAN><SPAN class="attr-value token"><SPAN class="punctuation token">=</SPAN><SPAN class="punctuation token">"</SPAN>true<SPAN class="punctuation token">"</SPAN></SPAN><SPAN class="punctuation token">/></SPAN></SPAN><SPAN class="line-numbers-rows"><SPAN></SPAN></SPAN>
and this seems to work fine for the original popup data, but, the related records are still removing a day from the date.
Has anyone figured out a way to have the related table date format honor the popup config file format or am I stuck with showing two different dates for that same field?

The data in the feature class and the related table both have 1/13/2016 in this field.
Thanks for any help,
R_