iPhone photo dates incorrect after transferring data from Android

Which date are you looking at and where?

Images have internal EXIF metadata date which is quite robust. On the other hand less important but usually more readily displayed filesystem file creation & modification dates are quite fragile and are easily changed when moving files around. This creates much confusion here.

If the latter, you can very quickly use 3rd party tools like GraphicConverter (below) or exiftool to copy EXIF metadata dates to those (not so important) file dates:

BUT many social media sites like WhatsApp, Facebook etc strip ALL metadata (dates, locations) from images for privacy reasons so it might be possible that your images don't have any meaningful dates so Photos.app grabs the file creation date as a last resort.

GraphicConverter or exiftool can reliably display internal metadata and verify if the EXIF dates are there or not. Or:

Internal image metadata date 'ExifIFD:DateTimeOriginal' can be checked outside Photos.app in Finder > View > Show Preview > Show More > 'Content Created' date in the right side panel (this is DateTimeOriginal but for some reason Apple labels it differently here). Screenshot on the left below.

Or in Preview.app > Tools > Show Inspector > i > Exif > 'Date Time Original' field. (FWIW sometimes Finder and Preview might display another date tag in those fields so I prefer to use GC or exiftool for that). Screenshot on the right below.

As Matti pointed out, the date you get depends on the transfer method. Image Management Systems like Photos use the Image Creation Date, but once the picture is in the "hands" of File Management Systems like Finder or Explorer, then everything can change.

Since you have your old phone, you may be better off starting all over again, and transferring your pictures directly to iCloud Photos. I have zero experience with this, by you can see this:

Move content manually from your Android device to your iPhone or iPad - Apple Support

The key may be to use the  Apple Devices app. You could start with just a few pictures and see if the method you choose preserves the dates. Personally, I'd rather find a way to move the picture, preserving dates, than having to change all the dates (again!)

Matti uses Windows ( I think,) so he may have better ideas about moving pictures around like this.

Wow-- you'd think that doing more operations would be more likely to mess things up! But convoluted is OK as long as it's not something that you have to do over and over. I'm glad this worked!