esp8266 deep sleep timer slightly inaccurate?
This (now 3 year old) github / esp8266 thread suggests using an external RTC if time tracking is of importance.  It also points at a dead NodeMCU document link.  That document appears to have move here.  This document describes the lengths taken to provide a better account for time passage during sleep.  Which, apparently, includes guessing at the sleeping chip's temperature: 


  Time keeping on the ESP8266 is technically quite challenging. Despite
  being named RTC, the RTC is not really a Real Time Clock in the normal
  sense of the word. While it does keep a counter ticking while the
  module is sleeping, the accuracy with which it does so is highly
  dependent on the temperature of the chip. Said temperature changes
  significantly between when the chip is running and when it is
  sleeping, meaning that any calibration performed while the chip is
  active becomes useless mere moments after the chip has gone to sleep.
  As such, calibration values need to be deduced across sleep cycles in
  order to enable accurate time keeping. This is one of the things this
  module does.