It is another day... and yet, it is not. (A truly unimpressive truism. But withholding expletives, you find it a tad difficult to cohesively — let alone eloquently — describe the particular situation.) For as soon as you opened your eyes, you knew that something was dreadfully odd.

Alas, from the lottery draw of life that deals out coincidences and misfortunes and fated encounters on the spectrum of arid to astounding, you have seemingly hit jackpot. Congratulations! You have won the most uncanny and inexplicable, even impossible (though obviously now disproven to be impossible, c.f.: current predicament) happenstance:

The reflection in the mirror is yours yet aged, the year on the calendar is alarmingly advanced.

This can't be real-life. A dream, perhaps? But you are certainly conscious, and everything is as tangible as it could be. 

The only explanation, that you will eventually settle on with an uneasy bewilderment after freaking out for a sufficient period of time, is that you have time-slipped into your adult self's body.