Unless they rounded up, there is a three month time span between s1e1 and s2e1. Since s2e1 is canonically the first day of spring, and assuming that Amphibian seasons are the same as Earth's, this is in April, the fourth month. This would also mean that the show as a whole takes place over the course of over a year. This is because the s3a finale is during Christmas, which is in December. I'm assuming that there was maybe 3 months between s3a finale and s3b pilot, seeing as the weather in Amphibia indicates Spring is in session. Add to that maybe another 1 or 2 months in Amphibia, and you've got got yourself nearly a year and a half of Amphibia.

I'm sure that if I was a bit more motivated or whatever I could a figure out an approximate date, depending on the month. Though this depends on the show being faithful to moon phases and whatnot, and I haven't paid nearly enough attention to it.