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Thunderbird

ISO Date

/edit/settings - bottom of page - config editor

Paste the ones below hit the +  Select the data type

Boolean
intl.date_time.pattern_override.connector_short and set it to true
String
intl.date_time.pattern_override.date_short and set it to yyyy-MM-dd
String
intl.date_time.pattern_override.time_short and set it to HH:mm