This is the code that applies your browser's current timezone offset to guess at the proper transformation to apply to a tideline date string in order to store a UTC ISO string as the `timestamp` of the new message. This code was previously in blip, but I had to move it to tideline because of the issue with formatting the date string properly for newly-created messages versus stored messages (the former were in tideline "fake" UTC, while the latter were the UTC we're storing with the messages).

Note that `getTimezoneOffset()` gives the offset in terms of the minutes of *difference* from UTC, so the sign is opposite to the sign of your stated offset. i.e., NZ is UTC+12 but an offset of -720, and SF is UTC-7 but an offset of +420. Hence the addition and not subtraction.