Describe the bug
setting color to a specific day is not working for a general diagram without start date

To Reproduce

@startgantt
[C] lasts 10 days
[C] starts at D+10

D+5 to D+7 are colored in blue
D+10 is colored in gold

footer \n(current version: %version())

@endgantt  

Expected behavior
mentioned days are colored

Screenshots
actual result for latest version

Additional context
the coloring works fine if i add a project start date (i.e. "project starts the 2021/01/10"), however this changes a project timeline as well. I would like to mark specific days without connecting project timeline to a calendar.