Military Time and Roman Numerals

Two systems that predate modern medicine remain embedded in everyday healthcare: the 24-hour clock (military time) and Roman numerals. Hospitals use military time to eliminate AM/PM confusion — a medication charted at “0200” can only mean 2:00 in the morning, while “2:00” could be misread as either AM or PM. Roman numerals appear on prescription labels, physician orders, and apothecary notation. Both are simple once you learn the conversion rules, but misreading either one can lead to medication timing errors.

Part 1: Military Time (24-Hour Clock)

Why Healthcare Uses Military Time

The 12-hour clock creates ambiguity. If a medication is ordered for “8:00,” a nurse working a double shift could administer it at 8:00 AM or 8:00 PM — a 12-hour timing error. Military time removes this entirely by numbering all 24 hours sequentially from 0000 (midnight) through 2359 (one minute before the next midnight). There is no AM or PM. Every time is unique.

Reading Military Time

Military time uses four digits with no colon (though some facilities include a colon in electronic charting). The first two digits are the hour (00 to 23), and the last two are the minutes (00 to 59).

• 0000 = midnight (12:00 AM)
• 0615 = 6:15 AM
• 1200 = noon (12:00 PM)
• 1345 = 1:45 PM
• 2359 = 11:59 PM

Converting Standard Time to Military Time

For AM hours (midnight through 11:59 AM):

1. Keep the hour number as-is.
2. Add a leading zero if the hour is a single digit.
3. Drop the “AM.”

Special case: 12:00 AM (midnight) becomes 0000, not 1200. And 12:30 AM becomes 0030 — the midnight hour uses 00, not 12.

For PM hours (noon through 11:59 PM):

1. For 12:00 PM through 12:59 PM, keep the number as-is: 12:00 PM = 1200.
2. For 1:00 PM through 11:59 PM, add 12 to the hour.
3. Drop the “PM.”

Converting Military Time to Standard Time

If the hour is 00 (0000 to 0059): Replace 00 with 12 and add “AM.”

• 0000 → 12:00 AM
• 0045 → 12:45 AM

If the hour is 01 through 11 (0100 to 1159): Drop the leading zero (if any) and add “AM.”

• 0700 → 7:00 AM
• 1130 → 11:30 AM

If the hour is 12 (1200 to 1259): Keep as-is and add “PM.”

• 1200 → 12:00 PM
• 1215 → 12:15 PM

If the hour is 13 through 23 (1300 to 2359): Subtract 12 from the hour and add “PM.”

• 1300 → 1:00 PM ($13 - 12 = 1$)
• 1800 → 6:00 PM ($18 - 12 = 6$)
• 2100 → 9:00 PM ($21 - 12 = 9$)

Military Time Quick Reference Table

Calculating Time Elapsed

To find how much time has passed between two military times, subtract the earlier time from the later time — but treat hours and minutes separately.

Example: A patient’s IV started at 0830 and finished at 1415. How long did the infusion run?

$14\text{h } 15\text{min} - 08\text{h } 30\text{min}$

Since 15 minutes is less than 30 minutes, borrow 1 hour:

$13\text{h } 75\text{min} - 08\text{h } 30\text{min} = 5\text{h } 45\text{min}$

Answer: The infusion ran for 5 hours and 45 minutes.

Part 2: Roman Numerals in Healthcare

The Seven Symbols

Roman numerals use seven letters, each with a fixed value:

The Rules

Rule 1 — Addition: When a smaller or equal numeral follows a larger one, add the values.

• VI = 5 + 1 = 6
• XV = 10 + 5 = 15
• VIII = 5 + 1 + 1 + 1 = 8

Rule 2 — Subtraction: When a smaller numeral comes before a larger one, subtract the smaller from the larger.

• IV = 5 - 1 = 4
• IX = 10 - 1 = 9
• XL = 50 - 10 = 40
• XC = 100 - 10 = 90

Rule 3 — Repetition: A symbol can repeat up to three times in a row.

• III = 3
• XXX = 30
• But you never write IIII for 4 — use IV instead.

Roman Numerals Commonly Seen in Nursing

In healthcare, you will most often encounter the small Roman numerals — lowercase versions used on prescriptions and medication orders:

Important note: The abbreviation “ss” for one-half is from the Latin “semis.” While its use is declining due to patient safety initiatives (it can be confused with “55”), you may still encounter it on older orders or in pharmacology textbooks.

Reading Prescription Notation

A traditional prescription might read:

Aspirin gr v tabs ii PO q4h PRN

This means: Aspirin, 5 grains, 2 tablets, by mouth, every 4 hours, as needed.

Breaking down the Roman numerals:

• gr v = 5 grains (grain is an apothecary weight unit; 1 grain = approximately 60 to 65 mg)
• tabs ii = 2 tablets

While modern prescriptions increasingly use standard numerals for safety, nurses must be able to read both formats.

Common Mistakes to Avoid

1. Confusing 0000 and 1200. Midnight is 0000, noon is 1200. A medication scheduled at “1200” is a midday dose. This is the most common military time error.
2. Adding 12 to the noon hour. 12:00 PM is already 1200 in military time. Adding 12 would give 2400, which is not standard. Only add 12 to PM hours from 1:00 PM through 11:59 PM.
3. Reading IV as “4” when it means “intravenous.” Context matters. In a Roman numeral context (e.g., “tabs iv”), IV means 4. In a route of administration (e.g., “give IV”), it means intravenous. Read the full order to determine meaning.
4. Misreading IX (9) as XI (11). Order matters in Roman numerals. I before X means subtract: IX = 9. I after X means add: XI = 11. Swapping them is an error of 2 units.
5. Forgetting to borrow when subtracting military times. When the minutes of the later time are smaller than the minutes of the earlier time, you must borrow 60 minutes from the hours — just like subtraction with regrouping.

Practice Problems

Test your understanding with these problems. Click to reveal each answer.

Key Takeaways

• Military time uses a 24-hour clock (0000 to 2359) to eliminate AM/PM confusion
• For PM hours 1:00 through 11:59, add 12 to the hour; for AM hours, use the hour as-is with a leading zero
• Midnight = 0000, noon = 1200 — do not confuse them
• Roman numerals use seven symbols (I, V, X, L, C, D, M) with addition and subtraction rules
• In healthcare, you most often see lowercase Roman numerals i through x and ss (one-half) on prescriptions
• Context determines whether “IV” means the number 4 or “intravenous” — always read the complete order
• When calculating elapsed time in military format, borrow 60 minutes from the hours when needed

Return to Math for Nurses for more topics.