What's a good wat to trunctate a time point

[Biotronic via Digitalmars-d-learn]

On Friday, 5 May 2017 at 09:14:21 UTC, Biotronic wrote:
> Here's an implementation that supports start of year, month, 
> week, day, hour, minute and second. Works for DateTime and 
> SysTime. Not heavily tested (all tests included):

As the last sentence says, there were holes in the testing, 
specifically for unsupported units. Updated:

import std.datetime;

DateTime startOf(string unit, DayOfWeek start = 
DayOfWeek.sun)(DateTime dt) {
     static if (unit == "year") {
         return DateTime(dt.year, 1, 1);
     } else static if (unit == "month") {
         return DateTime(dt.year, dt.month, 1);
     } else static if (unit == "week") {
         auto delta = dt.dayOfWeek - start;
         if (delta < 0) delta += 7;
         return DateTime(dt.year, dt.month, dt.day) - 
dur!"days"(delta);
     } else static if (unit == "day") {
         return DateTime(dt.year, dt.month, dt.day);
     } else static if (unit == "hour") {
         return DateTime(dt.year, dt.month, dt.day, dt.hour);
     } else static if (unit == "minute") {
         return DateTime(dt.year, dt.month, dt.day, dt.hour, 
dt.minute);
     } else static if (unit == "second") {
         return DateTime(dt.year, dt.month, dt.day, dt.hour, 
dt.minute, dt.second);
     } else {
         static assert(false, "\"" ~ unit ~ "\" is not a valid 
time unit for startOf().\nPlease use one of \"year\", \"month\", 
\"week\", \"day\", \"hour\", \"minute\" or \"second\"");
     }
}

SysTime startOf(string unit)(SysTime st) {
     return SysTime(startOf!unit(cast(DateTime)st), st.timezone);
}

unittest {
     auto now            = DateTime(2017, 5,  5, 10, 39, 17);

     auto expectedYear   = DateTime(2017, 1,  1,  0,  0,  0);
     auto expectedMonth  = DateTime(2017, 5,  1,  0,  0,  0);
     auto expectedWeek   = DateTime(2017, 4, 30,  0,  0,  0);
     auto expectedDay    = DateTime(2017, 5,  5,  0,  0,  0);
     auto expectedHour   = DateTime(2017, 5,  5, 10,  0,  0);
     auto expectedMinute = DateTime(2017, 5,  5, 10, 39,  0);
     auto expectedSecond = DateTime(2017, 5,  5, 10, 39, 17);

     auto startOfYear   = now.startOf!"year";
     auto startOfMonth  = now.startOf!"month";
     auto startOfWeek   = now.startOf!"week";
     auto startOfDay    = now.startOf!"day";
     auto startOfHour   = now.startOf!"hour";
     auto startOfMinute = now.startOf!"minute";
     auto startOfSecond = now.startOf!"second";

     assert(expectedYear   == startOfYear);
     assert(expectedMonth  == startOfMonth);
     assert(expectedWeek   == startOfWeek);
     assert(expectedDay    == startOfDay);
     assert(expectedHour   == startOfHour);
     assert(expectedMinute == startOfMinute);
     assert(expectedSecond == startOfSecond);

     now = DateTime(2017, 4, 30, 10, 39, 17);
     auto expectedWeek2  = DateTime(2017, 4, 24,  0,  0,  0);
     auto startOfWeek2   = now.startOf!("week", DayOfWeek.mon);
     auto expectedWeek3  = DateTime(2017, 4, 29,  0,  0,  0);
     auto startOfWeek3   = now.startOf!("week", DayOfWeek.sat);
     auto expectedWeek4  = DateTime(2017, 4, 30,  0,  0,  0);
     auto startOfWeek4   = now.startOf!("week", DayOfWeek.sun);

     assert(startOfWeek2 == expectedWeek2);
     assert(startOfWeek3 == expectedWeek3);
     assert(startOfWeek4 == expectedWeek4);

     assert(!__traits(compiles, now.startOf!"fortnight"));
}