Create utils for time manipulation on JS Date objects

[bjuppa]

It could be useful to provide utils for directly truncating minutes relative full hours directly on JS Date objects, and it's easy to forget that it requires a timezone to do correctly... ;)

ComPlainDate could help developers by directly providing:

• startOfHour(timezone: string)(instant: Date): Date
• startOfDay(timezone: string)(instant: Date): Date

...and perhaps also:

• truncateToHalfHour
• truncateToThirdOfHour
• truncateToQuarterOfHour
• truncateToSixthOfHour
• truncateToTwelfthOfHour