Does the time difference between Johannesburg and Mexico City change during the year?

No. The gap stays at 8h all year.

When is the best time to call Mexico City from Johannesburg?

There is no overlap between standard working days, so one side has to call early or late.

Johannesburg is 8h ahead of Mexico City. This gap is the same all year — neither city shifts its clocks relative to the other.

Johannesburg

South Africa

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Mexico City

Mexico

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A 9-to-5 working day in Johannesburg does not overlap one in Mexico City at all — somebody has to take the call outside office hours.

City	Time zone	UTC offset
Johannesburg · South Africa	Africa/Johannesburg	UTC+2
Mexico City · Mexico	America/Mexico_City	UTC−6

What is the time difference between Johannesburg and Mexico City?: Johannesburg is 8h ahead of Mexico City. This gap is the same all year — neither city shifts its clocks relative to the other.
Does the time difference between Johannesburg and Mexico City change during the year?: No. The gap stays at 8h all year.
When is the best time to call Mexico City from Johannesburg?: There is no overlap between standard working days, so one side has to call early or late.