תָּו Tav #8420 meaning mark (mark on forehead, as a sign of exemption from judgment) 

This word is only found in in three verses:  Job 31:35 (as a signature) and in Ezekiel 9:4 and 6.

In the New Testament we are given a seal of the Holy Spirit. (Eph. 1:13,14 / 2 Cor. 1:22)

Seal #G4972 σφραγίζω sfrag-id'-zo To set a seal upon, mark with a seal, to seal (for security: from Satan)

Some examples of covenants made, and signs given in the Old Testament are Noah- Gen. 9:17 / Abraham- Gen. 17:11 / Moses - Ex. 3:12/ Jesus - Isaiah 7:14.

Truth encompasses all things, from the beginning, through to the end.

Truth #571 emet אָמַן From the root word #539 aman  properly to build up or support, believe. If you remove the א (the Leader- God) you get the word dead.  מת With God there is truth, without God there is death. Prov. 14:12

Jesus is the truth of God.  John 1:14 / 14:6

Teshuvah is typically translated as repentance, but it literally means  return.  It is when one turns back toward something or someone that they have strayed or looked away from.

תשובה Shuv #7725 to return, turn back (with the idea of restoration) Joel 2:12,13 / Ez. 18:30 / Matt.4:17 / Romans 2:4

Tav is the last letter of the Hebrew alphabet, and it speaks of when things run their course, reached their goal and are ready to come to the end.  Rev. 22:13

Numeric Value 400 is a period of lament:

The Israelites were enslaved 400 years, Gen.15:13. 

A grieving Abraham bartered for a gravesite to bury Sarah. Gen. 23:15. 

The 400-year period of silence between the Old Testament and New Testament.