if i have the Key but i download the Encrypted video 1 or 2 month later

[Loryanam2]

will my key still work or it will expire or changed by widevine ?

also why when i decrypt my video with mp4decrypt using cmd
im Not able to fast forward unless i converted to mkv ?

[notaghost]

Yes they will work. if you have the same mpd + same keys. But there is a possibility they can rotate keys.

[[ss]vegeta]

if i have the Key but i download the Encrypted video 1 or 2 month later will my key still work or it will expire or changed by widevine ?

Depends on the site, but it should work. But why on earth would you download it 1-2 months later?

im Not able to fast forward unless i converted

Decrypted video that is merged with decrypted audio, should not have "fast forward" problems.
When you only have decrypted video, some players might have slight hiccups.
Try different players and make sure your video and audio are merged.

ffmpeg -i video_dec.mp4 -i audio_dec.aac -c copy finalvideo.mp4

I'm not sure about decrypted audio in an aac container though.