Functions to create a Period from a single dateTime

[Seddryck]

It should be possible to create a function from a single dateTime. The following functions should be read as dateTime-to-period-...

• starting-now : returns a period where the date argument is the end of the period and the beginning is now
• ending-now : returns a period where the date argument is the beginning of the period and the end is now
• starting-ago(timestamp) : returns a period where the date argument is the end of the period and the duration of the interval is equal to the timestamp.
• ending-in(timestamp) : returns a period where the date argument is the beginning of the period and the duration of the interval is equal to the timestamp.
• covering(timestamp) : returns a period where the date argument is the middle of the period and the duration of the interval is equal to the timestamp.
• starting-at(dateTime) : returns a period where the date argument is the end of the period and the date provided as parameter of the function is the beginning
• ending-at(dateTime) : returns a period where the date argument is the beginning of the period and the date provided as parameter of the function is the end