It looks like clock free scan recognizes literal "T" in (undocumented?) ISO 8601 timestamp incorrectly, I guess as a legacy (military?) timezone "T".
<pre style="padding-left:10pt">
# ISO:
% clock format [clock scan "20200520 14:40:00" -gmt 1] -gmt 1
Wed May 20 <b style="color:green">14:40:00</b> GMT 2020
% clock format [clock scan "20200520T14:40:00" -gmt 1] -gmt 1
Wed May 20 <b style="color:green">14:40:00</b> GMT 2020

# ISO-similar format:
% clock format [clock scan "2020-05-20 14:40:00" -gmt 1] -gmt 1
Wed May 20 <b style="color:green">14:40:00</b>:40:00 GMT 2020
% clock format [clock scan "2020-05-20T14:40:00" -gmt 1] -gmt 1
Wed May 20 <b style="color:red">21:40:00</b> GMT 2020
% clock format [clock scan "2020-05-20 14:40:00  T"    ] -gmt 1
Wed May 20 <b style="color:red">21:40:00</b> GMT 2020
</pre>
This seems not correct in my opinion (despite the documentation says about literal "T" by ISO without "-" separator in date only, recognize "T" as timezone looks weird to me).

I think either we must aggressively strict the timezones bison/yacc rules (must consider word boundaries or something similar) or ISO format must be extended with YYYY-MM-DD date.