there the attribute "billable" is of type boolean.

But it seems that it has been implemented not based on the value of the attribute, but the existance of the attribute in post. All the values:

'billable': False

'billable': 'False'

'billable': 'hapuoblikas'

'billable': 0

'billable': 1

produce the time entry as

'billable': True

The only way it seems to produce the time entry as

'billable': False

seems to be to omit the attribute from the post.