month and year values are not converted into Duration and lead to data loss

[nitram509]

Issue description

the month and year values are not converted and leads to data loss
see func (duration FEELDuration) Duration() time.Duration {...}

See also Gemeni's commet #26 (comment)

TODO

• FEELDuration must convert to time.Duration without any data loss
• check also the other direction time.Duration -> FEELDuration