It's very likely there are a bunch of photos in your Picasa photo collection that are displaying the incorrect date and time when the photos were taken.
And this isn't just a problem when your photos won't sort chronologically. This will also be an issue for you every time you create a new folder or album inside of Picasa and it constantly tries to identify them use the wrong date.
Maybe the date and time weren't set correctly in your digital camera before you took these photos. Or it's possible you scanned a bunch of prints or film negatives and they are still reflecting the dates and times when you actually scanned them.
Either way, you'll be happy to know as of version 3.5 of Picasa (changelog), you now have the ability to easily correct the date and time of your pictures and videos using the following steps.
How to Change the Date and Time Metadata in Picasa
The following instructions work for both the Windows and Mac versions of Google's Picasa.
Verifying the Changes Were Applied
You can see that your dates have been updated by viewing the metadata information listed below your photos and videos in the “Editing” view.
Additionally, you will see the corrected date listed in the Properties Panel on the righthand side. (Click the button on the bottom right marked with an “i” to open up this panel)
Depending on variables such as the file type (.jpg, .png, or .tiff e.g.) of the image you adjusted and the operating system and version you are running, the date may be listed as either the “Camera Date” and/or the “File Date” property.
The instructions and screenshots were created using Picasa v.3.9.0 on Microsoft Windows 7.