todayyoudaoicibaDictYouDict[today 词源字典]
today: [OE] Today is simply a compound assembled from the preposition to (in the now obsolete sense ‘at, on’) and day. Parallel formations are Dutch vandaag (literally ‘from or of day’) and Swedish and Danish i dag (‘in day’). In fact virtually all the terms for ‘today’ in the European languages contain an element meaning ‘day’, but not all of them are as obvious as today, vandaag, and i dag.

German heute, for instance, comes from a prehistoric Germanic *hiu tagu, which meant literally ‘on this day’. Russian segodnja likewise denotes etymologically ‘this day’. And the second syllable of Latin hodiē ‘today’ (ancestor of French aujourd’hui, Italian oggi, and Spanish hoy) represents an inflected form of diēs ‘day’. Tomorrow and tonight [OE] were formed on the same basis.
[today etymology, today origin, 英语词源]
today (adv.)youdaoicibaDictYouDict
Old English todæge, to dæge "on (this) day," from to "at, on" (see to) + dæge, dative of dæg "day" (see day). Meaning "in modern times" is from c. 1300. As a noun from 1530s. Generally written as two words until 16c., after which it usually was written to-day until early 20c.

Similar constructions exist in other Germanic languages, such as Dutch van daag "from-day," Danish and Swedish i dag "in day." German heute is from Old High German hiutu, from Proto-Germanic *hiu tagu "on (this) day," with first element from PIE pronominal stem *ki-, represented by Latin cis "on this side."