Related Questions:
Why am I unable to Post Charges to Mews?
What does "Invalid Start Time" mean?
Answer:
This means it cannot post because one of the selected Events is too far in the past. This happens if you are posting for something that ends outside your editable history interval
Example:
Editable history interval = 1 day
Today = Jan 6
Event end date = Jan 4
Consumption time of items posted will be Jan 4 @ 23:59
That is too far in the past