in which cases expression for `addTokens` can be negative (clocks skew) in lua script?

[kashif-ghafoor]

Question

I am trying to understand the implementation of token bucket in lua script. I cam across following expression and it's comment from rollinglimit.lua

-- tokens that should have been added by now
-- note math.max in case this ends up negative (clock skew?)
-- now that we call 'TIME' this is less likely to happen
local addTokens = math.max(((nowMS - lastUpdateMS) / intervalMS) * limit, 0)

comment mentions the use of math.max in cases where expression ((nowMS - lastUpdateMS) / intervalMS) * limit can be negative.

I believe expression can only be negative when lastUpdateMs is in future but that is unlikely happen without external change of value.

I want to understand and make sure the robustness of implementation. Can you specify In which cases expression can end up negative?
I searched about clock skew but it seems to be a low level circuit concept than a programming concpet.

Currently I have one redis instance. May be I am unware of sitation where expression can end up in negative.

Thank you for your time and effort.