Can I use an internal clock on Wemos D1 Mini?
I think the answer to your question can best be summed up by one word: &quot;Kinda&quot;.
The best reasoning is the NodeMCU Lua documentation which states:

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. [...]

So while it's called RTC it's not an RTC in the traditional sense - it's merely a timer that runs even when sleeping, but not with any degree of accuracy.
You can access the current RTC count with the Non OS SDK function
uint32_t system_get_rtc_time()

You can combine that with
uint32_t system_rtc_clock_cali_proc()

which gives you the number of uS per tick of the RTC timer.
There's no way to change the RTC timer count, so you will have to work out some &quot;delta magic&quot; to update your own internal time value (an epoch value is simplest) from the difference between successive calls to get the RTC count.
All somewhat tricky and the accuracy won't be great anyway, so you'll be wanting to use NTP to update that epoch value regularly.
But it could give you a &quot;rough idea&quot; of how long you've been asleep for, etc.
You would have to run some experiments to know how much the time drifts compared to the NTP value in real usage.