What does “time limited” mean here?
During that period the content can be recovered by using the masterkey to unwrap the data key and decipher the content. Beyond that period, if you don't remember the master key, you cannot decrypt the content.
The time-period is only a shortcut. So, for personal use, as the OP intended, resetting the masterkey each year would still offer some protection against forgetting the passphrase for the data encryption.
Could your time-based scheme be defeated by changing the system clock on the attacker computer?
In the following [] denotes a byte sequence and | denotes concatenation of two byte sequences and +/-> denotes a transformation.
Encryption
[correspondence] -> [masterkey encrypted datakey|datakey encrypted data]
Normal Decryption
[masterkey encrypted datakey|datakey encrypted data] + [datakey] -> [correspondence]
Key recovery (datakey lost/forgotten)
[masterkey encrypted datakey|datakey encrypted data] + [masterkey] -> [datakey]
Once the datakey is recovered, the encrypted data can be read.
We use keyrecovery as a backup.
We don't need to worry about clock accuracy for this as the timeframe we are talking about is in the order of days or months.
For added security you can use a different keyderivation/encryption method for the masterkey and even a different encryption method. For eg: You could use RSA to encrypt the datakey while you use AES for the data.
I hope this clarifies.