How to name a 15-minute period?

In Dutch, we have the word "kwartier" to denote a 15-minute period. It is derived from the word "kwart", which means quarter. It is very common to use this word in both spoken and written language.

Is there a similar word or expression in English (e.g. quarter)? And is it very common to use this word, or is it more commonplace to use expressions such as quarter of an hour, 15 minutes, etc.?

The scenario I'm considering in particular is that of labels on forms or in software. The 'user' has to indicate how much time was spent on something, in time units of 15 minutes. What would be the most appropriate way:

• Time spent: # quarters
• Time spent: # quarters of an hour
• Time spent: # times 15 minutes
• Time spent: # minutes (in software, make 15-minute increments the only possible way to input the time)
• …

Quarter-hour would be your best bet, although native speakers tend not to use this in regular conversation. Quarter of an hour is also correct, but, I daresay, less likely to be used. The phrase, every fifteen minutes can often be heard in everyday usage.

For the specific software situation you mentioned, again, quarter-hour would be your best bet. On its own, quarter in temporal contexts usually refers to quarter of a year.

"Quarters of an hour" or "15-minute periods" are the only ways I can think of. But an English speaker would be least confused by seeing a 'reduced fraction', as:

"Time spent: 2 hours and 15 minutes"

or

"Time spent: 1 hour and 45 minutes"

This might take a little extra coding, but seeing "Time spent: 12 15-minute periods" only makes sense if you always use it for one session, and one session is always 15 minutes.