Wrong date in list report when traveling between timezones

#### To report a bug...

* What command(s) did you run?
`task 1 modify scheduled:2018-06-08`
`env TZ='Europe/London' task list`
`env TZ='Europe/Moscow' task list`

* What did you expect to happen?
Display the same date in "Due" column no matter the timezone.
* What actually happened?
Different dates depending on TZ:
```sh
$ env TZ='Europe/Moscow' task list
ID Age Due Description Urg
1 6w 2018-06-08 buy bilk 7.03

$ env TZ='Europe/London' task list
ID Age Due Description Urg
1 6w 2018-06-07 buy bilk 7.03
```
* Paste the output of the ```task diag``` command.
```
~: task diag

task 2.5.1
Platform: Linux

Compiler
Version: 5.3.0
Caps: +stdc +stdc_hosted +LP64 +c8 +i32 +l64 +vp64 +time_t64
Compliance: C++11

Build Features
Built: Mar 9 2016 20:12:54
CMake: 3.4.3
libuuid: libuuid + uuid_unparse_lower
libgnutls: 3.4.10
Build type:

Configuration
File: /home/username/.taskrc (found), 1583 bytes, mode 100644
Data: /media/localStorage/backup/Linux/taskwarrior_data (found), dir, mode 40750
Locking: Enabled
GC: Enabled
$EDITOR: vim
Server:
Trust: strict
Certificate: , not readable, 0 bytes
Key: , not readable, 0 bytes
Ciphers: NORMAL
Creds:

Hooks
System: Enabled
Location: /media/localStorage/backup/Linux/taskwarrior_data/hooks
(-none-)

Tests
$TERM: xterm-termite (277x66)
Dups: Scanned 5 tasks for duplicate UUIDs:
No duplicates found
Broken ref: Scanned 5 tasks for broken references:
No broken references found
```
2018-06-08 is a valid ISO8601 date, but TW always adds the time component. It sometimes messes up dates when traveling between timezones.