walletFullDateTimeFormat function
The detail sheet's FULL date+time (yMMMd + jm — the sheet is where the
exact moment belongs; rows keep the compact form). Locale-aware, memoized.
Implementation
DateFormat walletFullDateTimeFormat(String locale) => _fullDateTimeFmts
.putIfAbsent(locale, () => DateFormat.yMMMd(locale).add_jm());