If you don't need exactly 45 days, but "one and a half months" will do, then a straightforward method would be to run at the beginning of the month every three months, and at the middle of the next month after each of those:
0 12 1 1,4,7,10 /path/to/script
0 12 16 2,5,8,11 /path/to/script
For general arbitrary intervals, the other answers are obviously better, but 45 days sounds like it's based on the length of a month anyway. Human users might also be more used to something happening in the beginning or the middle of a month, instead of seeing the exact date drift a day or two each time.
