Recently when reading some material related to research grants, I came across the Latin phrase pro rata temporis in English text. It was easy enough to understand in the context. For example, a 600 000 € grant might be for three years, and if one applies for a shorter period of time, the maximum money is reduced pro rata temporis, so that for two years one can get at most 400 000 €.
But how do I parse the Latin phrase? I have difficulties seeing how it could mean what it does. Is the phrase an abbreviation of something longer? Does it have an origin that would help me understand how it works? Would the phrase make sense in classical Latin without context? I take it that rata is a perfect participle of reri, and L&S lists under II.B.1 pro rata parte/portione, which makes me think that perhaps an ablative like parte or portione is implied.
