"Hear, O Israel! The LORD is our God, the LORD is one!" Deuteronomy 6:4
The Lord is one. One what?
The Hebrew word for "one" in this verse is ekhad. It is Strong's #259 and means "one". This word is used 712 times in the Tanakh. Sometimes it is translated as "one" and other times it is translated (in the NASB) as "first". The first time it is used is in Genesis 1:5-
God called the light day, and the darkness He called night. And there was evening and there was morning, one day.
Yom echad: one day. Here we see what is being counted: days.