The -d option only works on "normal" tshark output. When you use -T fields and select a particular time field, i.e. frame.time, then you get the format for your locale.
$env:TZ="UTC" tshark -r ... -T fields -e frame.time ...  should give you output in UTC. The -d -t option only works on "normal" tshark output. When you use -T fields and select a particular time field, i.e. frame.time, then you get the format for your locale. If you haven't set a TZ env variable, then tshark will use your "system" locale, so set TZ then run tshark, e.g. for PowerShell: $env:TZ="UTC"