esp8266 deep sleep timer slightly inaccurate?

Question

I'm making a door open timer using a hall sensor and have a wemos d1 mini that uses a deep sleep to sleep for 1 minute after checking the hall sensor state and printing the value on the serial port. I'm using the Arduino IDE to monitor the serial output and have the Show timestamp option enabled so that I could time how long the device is running after it resets. However, I'm getting output like this:

08:03:26.286 -> ^H⸮ȸ$RKx⸮⸮x⸮I⸮⸮696
08:04:25.454 -> HEA⸮⸮⸮d⸮⸮⸮⸮⸮⸮⸮696
08:05:24.681 -> ⸮⸮NI⸮ hlE⸮0⸮/⸮696
08:06:23.915 -> ⸮DP⸮V⸮l⸮X⸮X⸮H⸮⸮694
08:07:23.178 -> ⸮A⸮K`E⸮M⸮⸮xxE⸮696
08:08:22.406 -> ⸮HlX⸮K⸮I|⸮⸮xE⸮693
08:09:21.703 -> ⸮⸮NI⸮ hlE⸮0⸮⸮E⸮694
08:10:20.978 -> 8H⸮JcD⸮M⸮⸮xxE⸮695
08:11:20.306 -> ⸮⸮NI⸮AhlE⸮ Op⸮⸮696
08:12:19.573 -> ⸮⸮NI⸮ hlE⸮0⸮/⸮539
08:13:18.897 -> ⸮⸮OI⸮AhlE⸮⸮O⸮⸮538
08:14:18.196 -> HEA⸮⸮⸮d⸮⸮⸮⸮I⸮⸮539
08:15:17.561 -> ⸮dG⸮⸮⸮hLJ⸮@h⸮⸮(⸮539

The garbage output isn't what I'm asking about, it's the timestamps being created by the arduino ide. Since I sleep for 60,000,000 microseconds, I would assume that the second value of the minute would be slightly more each time. But as shown above, it becomes slightly less every time, as if it takes less than a minute. So what I'm wondering is if the sleep timer on these devices is not accurate enough to count on this or something else I should be considering like an inaccuracy in the IDE? I only wanted to measure the uptime so that I could consider it for battery drain considerations, I don't really need a millisecond accurate timer for the functionality of the device.

Answer 1

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.

Answer 2

Yes a RTC can be very inaccurate. Some RTCs will let you synchronise them.