The problem seems to be in the block log formatter.
BlockLogFormatter::getMessageParameters() calls
Language::translateBlockExpiry() to format the string "2 months" (and
translate it). That in turn uses strtotime() to parse this value, and
does not specify the $now parameter correctly, so January 1, 1970 is
assumed. And two months, counting from January 1, 1970, is 59 days. Two
months counting from today, when you made the block, is 61 days, and
this correctly appears everywhere else (the block expires
2017-01-04T06:30:06Z).
BlockLogFormatter::getMessageParameters() and
Language::translateBlockExpiry() should probably be fixed to use the
date when the block was made for the $now parameter of strtotime().
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Bartosz Dziewoński