2023
The Dating System
You aren't sure who decided it, but a new dating system was established in 2 PI. From what you remember, one of the last television channels airing announced it to the world one show prior to the television being abandoned (ever since, rouge persons with technical knowledge have hijacked some of the towers and signals and broadcast unofficial shows).
PI stands for Post Invasion. The number proceeding it simply declares the number of years after April 5th, 2010. Because of this, the old April 5th became the first day of the new type of year.
Months were never established by the show, so you can't be sure how the rest of the world count their progress through the year, but in Metropolis, you use a percent through the year. For example, today is 84 days since the beginning of 13 PI, so the current day is 22% 13 PI. The time of day works the same way. Right now it is 74% 22% 13 PI.