What is the oldest hymn (with music) that is still sung in churches today? Using any of the standard hymnals, what is the oldest hymn that people are still singing?

For example:


The Baptist Hymnal 1975 / 1991 / 2008, 
The Faith We Sing (Methodist, right?), 
The (Episcopal) Hymnal 1982, 
Adoremus (or anything else Catholics use) 
or just about any standard denominational hymnal in current usage


Alternatively, I'll take anything on Hymnary.org, since my aim is to find old hymns while excluding extremely obscure ones that only appear, for example, in a random hymnal from the 1700s.

Let me strike out one answer real quick – yes, I know the Psalms is a Jewish hymnbook.  The problem that I have with including the Psalms in the scope of this question is that we don't have the music :)

I suspect somebody can find obscure hymns that are old, but I'm looking for old hymns that are still sung widely. Things like the Doxology, the Sanctus, or other parts of the Mass would be likely candidates: as long as the song is still sung widely, I'm fine with translations of the original lyrics, but I'd prefer that the original tune/music be at least known, if not used.

Please include the date of the text and the music, if you could.

E.g., "All Creatures of Our God and King", 
words by Francis of Assisi (1200s), 
tune is Lasst Uns Erfreuen, 1623.
Probably the oldest text you'll find is the Phos Hilaron, sometimes known as "Hail gladdening light".  The earliest music for it was a Byzantine Chant, and it's still largely sung in churches worldwide, though not so much in the West.  

David Crowder released a version on his album "Church Music".  

Hail, gladdening Light, of His pure glory poured

Phos Hilaron (Wikipedia)

AFAIK, that's the oldest Christian hymn known today.