lose track of time

(verb) to be unaware of what time it is

Example Sentences:

Every time Matthew goes to the library, he loses track of time. Once, he stayed there for 8 hours!

A: Why are you so late?
B: I’m sorry! I was at work and I lost track of time!

I bought my husband a watch, because he was always losing track of time. Now he is never late.

Similar idioms:

1. lose track of
2. one-track mind
3. lose it