created localStringToZeroUTCOffsetString to convert local yyyy-mm-dd datestrings to yyyy-mm-dd datestrings with a zero UTC offset, which we now use before sending the date to the server.
The toISOString() method returns a string in simplified extended ISO format (ISO 8601), which is always 24 or 27 characters long (YYYY-MM-DDTHH:mm:ss.sssZ or ±YYYYYY-MM-DDTHH:mm:ss.sssZ, respectively). The timezone is always zero UTC offset, as denoted by the suffix "Z".
Changed the subtext of the Symptom Onset Date input to "Timezone set to " i.e.
localStringToZeroUTCOffsetStringto convert local yyyy-mm-dd datestrings to yyyy-mm-dd datestrings with a zero UTC offset, which we now use before sending the date to the server.Makes use of toISOString
closes https://github.com/covidwatchorg/portal/issues/443