# 4.10 24-Hour Clock

When reading or recording time according to the 24-hour clock, the hours are counted from one to twenty-four, and the minutes from one to fifty-nine. The 24-hour clock is used in all areas of health care to avoid confusion and errors.

There are always four digits in the number when stating the time using a 24-hour clock. The first two digits indicate the hour, and the second two digits indicate the minutes. The use of a.m. and p.m. is unnecessary. A few examples and explanations are given below:

- Morning hours are 0000 hours to 1200 hours. Note that 0000 hours (zero-hundred hours) is midnight, and 1200 hours (twelve-hundred hours) is noon.
- Afternoon hours are 1200 hours to 2400 hours. Note that 1200 hours (twelve-hundred hours) is noon, and 2400 hours (twenty-four-hundred hours) is midnight.
- 1000 (ten-hundred hours) means 10 o’clock in the morning.
- 2230 hours (twenty-two-thirty hours) means 10:30 in the evening.

Fig. 4.23 is an image of a 24-hour clock. Midnight would be 2400, and five minutes after midnight would be written as 0005. All the standard times with the corresponding 24-hour clock times are listed in Tables 4.39 and 4.40.

** Table 4.39. 24-Hour Clock (1 a.m. – Noon)**

STANDARD TIME |
24-HOUR CLOCK |

1:00 a.m. | 0100 hours |

2:00 a.m. | 0200 hours |

3:00 a.m. | 0300 hours |

4:00 a.m. | 0400 hours |

5:00 a.m. | 0500 hours |

6:00 a.m. | 0600 hours |

7:00 a.m. | 0700 hours |

8:00 a.m. | 0800 hours |

9:00 a.m. | 0900 hours |

10:00 a.m. | 1000 hours |

11:00 a.m. | 1100 hours |

12:00 noon | 1200 hours |

**Table 4.40. 24-Hour Clock (1 p.m. – Midnight)**

STANDARD TIME |
24-HOUR CLOCK |

1:00 p.m. | 1300 hours |

2:00 p.m. | 1400 hours |

3:00 p.m. | 1500 hours |

4:00 p.m. | 1600 hours |

5:00 p.m. | 1700 hours |

6:00 p.m. | 1800 hours |

7:00 p.m. | 1900 hours |

8:00 p.m. | 2000 hours |

9:00 p.m. | 2100 hours |

10:00 p.m. | 2200 hours |

11:00 p.m. | 2300 hours |

12:00 midnight | 2400 or 0000 |

(Snow, 2016)

**Roman Numerals**

Lowercase Roman numerals are occasionally used in medical terminology. You will most frequently see Roman numerals used in medication orders documentation and possibly for some laboratory tests.

**Lowercase Roman Numerals**

Table 4.41 lists lowercase Roman numerals up to 10. You will most likely see the numerals **i** to **iii** used in medical orders.

**Table 4.41. Roman Numerals**

STANDARD NUMBER |
LOWERCASE ROMAN NUMERAL |

1 | i |

2 | ii |

3 | iii |

4 | iv |

5 | v |

6 | vi |

7 | vii |

8 | viii |

9 | ix |

10 | x |

Key Concepts

**Examples of orders:**

- Polysporin eye gtts i–ii gtts in each eye bid at 0800 and 2000.
- Ventolin nebs x ii tid (0600, 1400, and 2200).

**Explanation of orders:**

- Polysporin eye drops, one to two drops in each eye twice a day at 8 o’clock in the morning and 8 o’clock in the evening.
- Ventolin two nebules three times a day at 6 o’clock in the morning, 2 o’clock in the afternoon, and 10 o’clock in the evening.

Exercise

**Attribution**

Unless otherwise indicated, material on this page has been adapted from the following resource:

Carter, K., & Rutherford, M. (2020). *Building a medical terminology foundation.* eCampusOntario. https://ecampusontario.pressbooks.pub/medicalterminology/ licensed under CC BY 4.0

**References**

Snow, M. A. (2016, January 8). *Nursing assistant: 24 hour clock time* [Video]. YouTube. https://www.youtube.com/watch?v=B3sPaYs24Oo

**Image Credits (images are listed in order of appearance)**