clock tampering
In order to obtain the access required to tamper with the clocks, one would first have to obtain access to the entire website. And as @Martin_Stahl once said, if they had that access, they would be doing much bigger things than tampering with the clocks.
The source of your problem is most likely lag or internet issues. If I am not mistaken, whatever time issues you suffer from lag or bad internet, is also suffered by your opponent in order to make it fair (I'm not completely sure about that one, but I seem to recall hearing it from a site moderator).
Perhaps try playing in a different area of your house? Or checking your internet? I know very little about internet connection, but I'm almost certain that your opponent is not changing the clocks.
Hope this helps!
Your opponent can't impact your connection.
Some of the information in the following may help:
https://support.chess.com/article/213-how-do-i-fix-my-disconnect-lag-issues
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The source of your problem is most likely lag or internet issues. If I am not mistaken, whatever time issues you suffer from lag or bad internet, is also suffered by your opponent in order to make it fair (I'm not completely sure about that one, but I seem to recall hearing it from a site moderator).
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The site tries not to pair a member with a good connection with someone with a poor connection, but once a game starts, if one member's connection starts having issues, that doesn't impact the other player's connectivity or clock.
https://support.chess.com/article/423-why-did-the-clock-times-suddenly-change-the-clocks-seem-broken
Definitely internet problem, had the same problem January. Problem was shared address, once returned to single use everything sped up and no more clock issues. (that was after at least 8 lost games down to time 
) I hope that this helps.
It's much more likely that your internet is cutting off than you playing against some mastermind doing the impossible.
Are you playing on Wifi?
Clock tampering doesnt exist on chess.com
if ur having clock issues its because of ur connection to the servers.
You didn't have second left, or rather you had a second during the premove but connection issues prevented the move making it to the server before that time had expired.
hehe, if you can tamper this, why not make it so that your clock never moves down and your opponents clock wastes double the amount of time
Why do so many people lose on bad connection and proceed to claim their opponent was tampering with their clock? OP didn't assert this so overtly, but I've seen others post vicious messages accusing their opponent of doing this. People need to understand that this is bad connection, which most people (myself included), seem to have. I apologize to whoever I'm posting after, as posts are coming in faster than I can type this one. I believe it's still a relevant question, though.
hehe, if you can tamper this, why not make it so that your clock never moves down and your opponents clock wastes double the amount of time
+1