15th day, 1st month
15th day, 2nd month
3rd month, 1st year
1st month, 2nd year
2nd month, 2nd yearEvent
Israel left Egypt as dawn broke
Israel came to the edge of the wilderness of Sinai
Israel entered the wilderness of Sinai
The construction of the Tabernacle began
The Tabernacle was completed, and Israel was numbered
The nation was only 14 months into its journey when Moses numbered the people. Since the number of firstborn sons is given as 22,273, then it would also represent the number of new fathers, i.e. ~4% of the male population 20+ years old. So, in all probability these are the firstborn sons delivered during the 14 month journey thus far.
The crude birth rate represented by the population numbers given here, is approximately 37 per 1000 during the 14 months, but when adjusted for 12 months, it is around 32 per 1000. This birth rate reflects what is said about the Hebrews early in Exodus:
7And the children of Israel were fruitful, and increased abundantly, and multiplied, and waxed exceeding mighty; and the land was filled with them.
19And the midwives said unto Pharaoh, Because the Hebrew women are not as the Egyptian women; for they are lively, and are delivered ere the midwives come in unto them.
-- Exodus 1:7,19
By way of comparison, that's 2.5 times the birth rate in Australia in 2015. Here's a link to crude birth rates (per 1000) around the world in 2015. Australia's rate in 1960 was 22 per 1000, so it has almost halved within the last two generations.
The 22,273 firstborn sons are those that were born after Israel left Egypt. The nation was born as the 14th day of Abib became the 15th at the beginning of the 1st year of its existence, and these 22,273 sons were the firstborn of the new nation.