Ok If I recall correctly Maggie was 18 in the first season then the second season comes and all sudden she's 20. I know in the USA school system kids born after August 31st are held back an school year because of an cutoff and Maggie was born September/October, 2001 so would it be safe to say the series is in 2021. I know the excate dates aren't know, but seeing as The Season 2 premier had Maggie celebrating her birthday it seems like Charmed is alot closer to 2022.
I think that it was just a simple retcon, unless stated otherwise you have to presume that the show is set at present. The writers have proven time after time again that they really don't give a flip about continuity or explaining things.
Maggie's birthday? Since the Synopsis for Safe Space reads
SEASON PREMIERE — Following the devastating loss of The Elders, Mel, Maggie and Macy take over their duties. Maggie takes time to celebrate her birthday with a big bash, but Mel finds herself too preoccupied with their new responsibilities to join in the fun.
However I still think people shouldn't being putting down 2019 when it comes to editing anything In Series for Season 2 related stuff. Since it might get confusing for people who are new fans and whatnot.