How do I store a period of time

[OscarCat]

How do I enter and store a period of time that may be of any length: years (.. unlikely but ...), days, hours, minutes, seconds (or even micro seconds) that I can later add to other dates or times stored in my database? How do I create a sensible data entry mask for yyyy/ddd/hh:nn:ss.999 and then convert this to a field compatible with date/time functions? And what format should this data take (be converted to ..?) and be stored in?
Or is this all somewhere sensible in a manual I don't have?

Any assistance appreciated.

[June7]

Options I see:
1. Store each time part in its own field. Then when you do calcs will have to convert each part to a common unit, such as days. Complication with this is that years don't all have the same number of days, this will have to be accounted for in the conversion.

2. Store single value as number of decimal days.

[OscarCat]

Thanks Super.

Painful. But I thought that might be the case.