Description:
When I used Calc to add up logged hours, I noticed some inaccuracies in the way LibreOffice formats and adds time values.

Steps to Reproduce:
- Format any field as a time value with format '[UU]:MM'
- Enter the value '01:55' or '2305:11' in the field (other values may behave the same).

Actual Results:
01:55 will be displayed as 01:54 in the spreadsheet, but the input line correctly shows 01:55:00.
2305:11 will be displayed as 2305:10, and the input line shows 2305:10:59.

When using these values in formulas later on, I get some unexpected results as well. For example adding 26:07 and 1763:35, with the former being the sum of 01:55 and some other values, results in 1789:41.

Expected Results:
All time values should be displayed and calculated as entered.


Reproducible: Always


User Profile Reset: Yes


OpenGL enabled: Yes

Additional Info:
Version: 6.2.0.3
Build ID: 6.2.0-3
CPU threads: 8; OS: Linux 5.0; UI render: default; VCL: gtk3; 
Locale: nl-NL (en_US.UTF-8); UI-Language: en-US
Calc: threaded