I'm not sure of the particulars but I remember someone telling me that Krishna or Vishnu is the very embodiment of the concept of time or something like that. Is it true?
Yes it is true.
In Chapter 11 of Bhagavad Gita Lord Krishna says he is Time.
kālosmi lokakṣayakṛt pravṛddho
lokān samāhartum iha pravṛttaḥ
ṛtepi tvāṃ na bhaviṣyanti sarve
yevasthitāḥ pratyanīkeṣu yodhāḥ 11.32
The Blessed Lord said:
Time I am, destroyer of the worlds, and I have come to engage all people. With the exception of you [the Pandavas], all the soldiers here on both sides will be slain.
Same thing is said in the Chapter 10 Vibhuti Yoga of Bhagavad Gita.
pralhādaś cāsmi daityānāṃ kālaḥ kalayatām aham
mṛgāṇāṃ ca mṛgendrohaṃ vainateyaś ca pakṣiṇām 10.30
Among the Daitya demons I am the devoted Prahlada; among subduers I am time; among the beasts I am the lion, and among birds I am Garuda, the feathered carrier of Visnu.
In Vishnu Purana Book 1 Chapter 2 Parashara says Time (Kala) is a form of Vishnu and it is through his forms of Kala and Purusha (Spirit) does Vishnu perform his actions.
He then existed in the forms of Purusha and of Kála. Purusha (spirit) is the first form, of the supreme; next proceeded two other forms, the discrete and indiscrete; and Kála (time) was the last.
These four--Pradhána (primary or crude matter), Purusha (spirit), Vyakta (visible substance), and Kála (time)--the wise consider to be the pure and supreme condition of Vishnu. These four forms, in their due proportions, are the causes of the production of the phenomena of creation, preservation, and destruction.
Vishnu being thus discrete and indiscrete substance, spirit, and time, sports like a playful boy, as you shall learn by listening to his frolics.