Wrong datetime calculation (A - B + B != A)

Bug description

• OS: Linux
• OS Version: Ubuntu 20.04
• Architecture: amd64

Unreleased version, commit 9ff8b0068411e9c1c5a414d1c70a3e963b2505e3, branch tsafin/gh-6751-olson-2

Tarantool 2.11.0-entrypoint-54-g9ff8b0068
Target: Linux-x86_64-Debug
Build options: cmake . -DCMAKE_INSTALL_PREFIX=/usr/local -DENABLE_BACKTRACE=ON
Compiler: /usr/bin/cc /usr/bin/c++
C_FLAGS: -fexceptions -funwind-tables -fno-common -fopenmp -msse2 -std=c11 -Wall -Wextra -Wno-strict-aliasing -Wno-char-subscripts -Wno-format-truncation -Wno-gnu-alignof-expression -fno-gnu89-inline -Wno-cast-function-type -Werror
CXX_FLAGS: -fexceptions -funwind-tables -fno-common -fopenmp -msse2 -std=c++11 -Wall -Wextra -Wno-strict-aliasing -Wno-char-subscripts -Wno-format-truncation -Wno-invalid-offsetof -Wno-gnu-alignof-expression -Wno-cast-function-type -Werror

Steps to reproduce

tarantool> dt2 = datetime.now()                                              
---           
...
                                      
tarantool> dt = { year = 1, month = 5, day = 17 }                            
---
...        

tarantool> dt2 = datetime.now()                                              
---
...

tarantool> sub_dt = dt2 - dt
---
...

tarantool> add_dt = sub_dt + dt
---
...

tarantool> add_dt == dt2
---
- false
...

tarantool> add_dt
---
- 2022-05-18T00:35:39.026203+0300
...

tarantool> dt2
---
- 2022-05-17T18:35:39.026203+0300
...

tarantool> 

Actual behavior

Expression has a wrong result (false).

Expected behavior

Expression should have a correct result (true).