Opening hours

Instead of using the graphical interface, you can enter and edit the opening hours directly using the OpenStreetMap syntax. Examples:

Mo-Fr 09:00-17:00
Open every Monday, Tuesday, Wednesday, Thursday and Friday from 9 (9 AM) to 17 (5 PM).
Mo-Th 09:30-17:00; Fr 10:00-15:00
Open every Monday, Tuesday, Wednesday, Thursday from 9:30 (9:30 AM) to 17 (5 PM) and every Friday from 10 (10 AM) to 15 (3 PM).
Mo-Fr 09:00-17:00; We off
Open every Monday, Tuesday, Thursday, Friday from 9 (9 AM) to 17 (5 PM), closed on Wednesday.
Mo,We 09:00-17:00; Tu,Th-Sa 10:00-12:00,13:00-15:00
Open every Monday and Wednesday from 9 (9 AM) to 17 (5 PM) and every Tuesday, Thursday, Friday and Saturday from 10 (10 AM) to 12 (12 PM) and then from 13 (1 AM) to 15 (3 PM).
Mo-Fr 09:00-17:00; Dec 24-26,Jan 1 off
Open every Monday, Tuesday, Wednesday, Thursday and Friday from 9 (9 AM) to 17 (5 PM), except for December 24, 25, 26 and January 1, when it's always closed.
24/7
Always open.
Mo-Su 20:00-02:00
Open every day from 20 (8 PM) to 2 (2 AM).
Fr 21:00-02:00, Sa 20:00-02:00
Open every Friday from 21 (9 PM) to 2 (2 AM) and every Saturday from 20 (8 PM) to 2 (2 AM).
Mo-Fr 09:00-12:00, Mo-Fr 13:00-16:00 "Payments only"
Open every Monday, Tuesday, Wednesday, Thursday and Friday from 9 (9 AM) to 12 (12 PM) and additionally from 13 (1 PM) to 16 (4 PM) with a comment "Payments only".

As you can see, the opening hours consist of rules separated by semicolons. You type a list (like Mo,We,Fr) or a range (like Mo-Fr) or even both (Mo-We,Fr) specifying the days, then you add one of more opening hours (using the 24-hour clock system, so 00:00 instead of 12:00 AM, 09:30 instead of 09:30 AM, 12:00 instead of 12:00 PM, 18:00 instead of 06:00 PM etc.) separated by commas. If that's not enough, add another days and corresponding opening hours after a semicolon (remember that the next rules override previous rules, if they both apply to the same day). Finally, you may add some special rules like Dec 24-26 off, which apply on specific days (and override the previous rules). You can also add a comment to each rule. But in this case I recommend specifying the days in each rule segment (after the comma) - use this: Mo-Fr 09:00-12:00, Mo-Fr 13:00-16:00 "Payments only"
instead of this: Mo-Fr 09:00-12:00,13:00-16:00 "Payments only". See the Common mistakes section.

You can also check out this simple tutorial:
Opening hours tutorial

If you're on a PC (it doesn't work on mobiles / tablets), you can also check out this nice web application called YoHours by Adrien Pavie.

Common mistakes

Even though the following opening hours are valid, they are most likely not what you want.

Mo-Fr 09:00-17:00, Th 10:00-16:00
This means that it's open from 9 (9 AM) until 17 (5 PM) each working day and additionally it's open from 10 (10 AM) until 16 (4 PM) on thursdays. The second part of the rule is useless in this case, since the Thursday "time range" is already included in the workday time range and therefore it doesn't add any new information. If you want to specify that on Thursday it's only open from 10 (10 AM) until 16 (4 PM), you must use a semicolon instead of a comma: Mo-Fr 09:00-17:00; Th 10:00-16:00. Use comma only if you want to express that some day (or days) have additional opening hours. For example Mo-Fr 09:00-12:00, Tu,Th 13:00-16:00 means it's open every working day from 9 (9 AM) to 12 (12 PM) and additionally on tuesdays and thursdays it's open from 13 (1 PM) to 16 (4 PM).

Dec 25,26 off; Mo-Fr 09:00-17:00
The second rule overrides the first rule, so the first rule it's useless and therefore these opening hours are the same as just Mo-Fr 09:00-17:00. You should always put the more specific rules after the less specific like this: Mo-Fr 09:00-17:00; Dec 25,26 off.

Fr 20:00-02:00; Sa 19:00-02:00
Again the second rule overrides the first rule. In this case both the rules define opening hours for Saturday - the first rule says it's open from midnight to 2 (2 AM) and the second rule says it's open from 19 (7 PM) to midnight. The second rule overrides the first, so it means that on Saturday it's open only from 19 (7 PM) to midnight and not from midnight to 2 (2 AM). In this case there should only be one rule with it's parts separated by a comma like this: Fr 20:00-02:00, Sa 19:00-02:00.

Mo-Fr 09:00-12:00,13:00-16:00 "Payments only"
Remember that if there are multiple rule segments (separated by commas) with days specified only at the beginning and a comment at the end, then that comment applies to all previous intervals (in that rule). So the opening hours above mean that both from 9 (9 AM) to 12 (12 PM) and from 13 (1 PM) to 16 (4 PM) it's only open for payments. In this case you should specify the days again after the comma like this: Mo-Fr 09:00-12:00, Mo-Fr 13:00-16:00 "Payments only".

Mo-Fr 09:00-12:00 "Payments only", 13:00-16:00If there are multiple rule segments (separated by commas) with days and comment specified only at the beginning, then all the following segments don't care about the previous days or comments. That means the opening hours say that not only is it open each working day in the morning (only for payments), it's open every day - including Saturday and Sunday - in the afternoon too. The correct string should be: Mo-Fr 09:00-12:00 "Payments only", Mo-Fr 13:00-16:00.