Task active time is not calculated based on start-Timestamp

Sometimes it makes sense to specify a `start` timestamp different from "now". For example when one worked for 30 min on something and then uses `modify start:-30min` to start the task 30 min in the past.
The task duration is not calculated correctly on these entries.

For example I just ran `task 123 mod start:18:10-1d && task 123 done` which resulted in
```
2019-09-11 19:40:00 Start deleted (duration: PT0S).
End set to '2019-09-11 19:40:00'.
Status changed from 'pending' to 'completed'.
```

zero seconds. This is incorrect.

I assume that the wrong task attribute is used to calculate the duration of a task.

#### Reproduction

* Example command:
```
task add Do the thing && task 123 mod start:-30min && sleep 2 && task 123 done
```

* What did you expect to happen?
The task has a calculated duration of 30 minutes and 2 seconds.

* What actually happened?
The task has a calculated duration of 2 seconds.

* Paste the output of the ```task diag``` command.

```
task 2.5.1
Platform: Linux

Compiler
Version: 8.2.0
Caps: +stdc +stdc_hosted +LP64 +c8 +i32 +l64 +vp64 +time_t64
Compliance: C++11

Build Features
CMake: 3.13.4
libuuid: libuuid + uuid_unparse_lower
libgnutls: 3.6.6
Build type: None

Configuration
File: /home/user01/.taskrc (found), 4096 bytes, mode 100644
Data: /home/user01/.task (found), dir, mode 40755
Locking: Enabled
GC: Enabled
Server:
Trust: strict
Certificate: , not readable, 0 bytes
Key: , not readable, 0 bytes
Ciphers: NORMAL
Creds:

Hooks
System: Enabled
Location: /home/user01/.task/hooks
(-none-)

Tests
$TERM: xterm-256color (206x115)
Dups: Scanned 1024 tasks for duplicate UUIDs:
Found duplicate 21f10f57-97d0-46c8-8e08-2faa3c76b8cb
Broken ref: Scanned 1024 tasks for broken references:
No broken references found
```
Apparently I generated the same UUID twice? I don't think that's possible...

Meanwhile I'm used to this specific bug. It's dangerous to get used to incorrect program behaviour.