"Bastard. (BrE) Mummy said it!"Before she (orig. AmE) outs me as a (orig. AmE) potty mouth at her (AmE) daycare/(BrE) crèche (or nursery), I'll have to take the matter into hand and save my sparkling wit (in response to Better Half's all-too-accurate parodies of me) for (BrE) after the watershed.
Because it's late at night (or early in the morning), I'll let Wikipedians do the work for me:
The term is an extension of other uses of watershed: 'the ridge or crest line dividing two drainage areas; water parting; divide' (which some dictionaries list as 'Chiefly BrE') and later ' an important point of division or transition between two phases, conditions, etc.'
United KingdomAccording to Ofcom, the watershed on standard television in the UK starts at 9:00 p.m., and finishes at 5:30 a.m. the next morning. Programmes that are 15+ are shown during this period. However, some 12+ shows can be shown before 9:00 p.m., such as The Simpsons, Malcolm in the Middle and Doctor Who. On premium film or pay-per-view services requiring a subscription, the watershed starts at 8:00 p.m. However, 12, 15 and 18 rated films can be shown on PIN protected channels (such as Sky Movies) at any time of the day. Viewers are required to enter their PIN to view. There should be a gentle transition to adult material, and 18-rated material is not allowed to be shown before 9:00 p.m.
See also for the UK: The Ofcom Broadcasting Code - Section 1
United StatesThe term "watershed" is not used in this context in the United States. In the US, the "safe harbor" for "indecent" programming begins at 10:00 p.m. and ends at 6:00 a.m. the next morning (all time zones). However, content that is considered "obscene" (including explicit human sexual intercourse) is never allowed by the FCC rules for broadcast stations. Those content rules only apply to channels broadcast terrestrially and not those only available on cable. Consequently, restricted-access networks (like the premium channels HBO and Showtime and adult channels Playboy TV and Spice) have taken advantage of considerably more leeway in their programming.
If I needed an equivalent for after the watershed in AmE, I think I'd say not in prime time, which isn't exactly the same thing. The watershed is a dividing line between the times when stricter and looser 'decency' codes have to be followed, whereas prime time is the part of the evening in which television networks expect to have the most viewers and therefore where they put their choicest programming (8:00 to 11:00 or 7:00 to 10:00, depending on the time zone). It's also when they charge the most for advertising time. In BrE, this is more commonly known as peak time, though since the major broadcaster (the BBC) is (orig. and principally AmE) commercial/(BrE)advert-free, it's less directly about advertising revenue. While prime time is not the only time when children might be watching, not in prime time is often used to mean 'not appropriate for a general audience'. This gives a double meaning to the name of Saturday Night Live's original troupe, the Not Ready for Prime Time Players.
On American (chiefly AmE) network television (i.e. distributing programs to local affiliates; not cable/satellite), the rules are fairly restrictive at all times, so I was surprised when I first moved to the UK and saw things like Something for the Weekend (which was really horrid) or The Sex Inspectors (experts watch couples getting it on and give them pointers on improving! The website describes it as post watershed), right there on free TV at a time when the equivalent US stations are showing the nighttime news. (Did the US ever import this format?) The reason why most of the good American television comes from HBO and Showtime is that those, as pay channels, do not have the same content restrictions as their free broadcast counterparts (and they've decided to use that power for good rather than evil).
At any rate, either Better Half will have to wait until the watershed from now on before he points out my pedantries and hypocrisies, or I'll have to rein in my tongue-in-cheek responses. Or else Grover will be teaching the entire pre-nursery room some choice AmE phrases. I think I know which one is most likely.