Issue #15772 has been reported by osyo (manga osyo).

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Feature #15772: Proposal: Add Time#ceil
https://bugs.ruby-lang.org/issues/15772

* Author: osyo (manga osyo)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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## Summary

proposal for a method that rounds up the decimal part of `Time` (nanosecond) to the specified digit.


## Current status

* `Time#round` and `Time#floor` (https://bugs.ruby-lang.org/issues/15653) are defined.
* However, `Time#ceil` is not defined.
* So, I would like to respond flexibly when I would like to control decimal precision.


## Propose `Time#ceil`

Below is example code.

```ruby
require 'time'

t = Time.utc(2010,3,30, 5,43,"25.0123456789".to_r)
p t.iso8601(10)          #=> "2010-03-30T05:43:25.0123456789Z"

p t.ceil(0).iso8601(10)  #=> "2010-03-30T05:43:26.0000000000Z"
p t.ceil(1).iso8601(10)  #=> "2010-03-30T05:43:25.1000000000Z"
p t.ceil(2).iso8601(10)  #=> "2010-03-30T05:43:25.0200000000Z"
p t.ceil(3).iso8601(10)  #=> "2010-03-30T05:43:25.0130000000Z"
p t.ceil(4).iso8601(10)  #=> "2010-03-30T05:43:25.0124000000Z"
p t.ceil(5).iso8601(10)  #=> "2010-03-30T05:43:25.0123500000Z"
p t.ceil(6).iso8601(10)  #=> "2010-03-30T05:43:25.0123460000Z"
p t.ceil(7).iso8601(10)  #=> "2010-03-30T05:43:25.0123457000Z"
p t.ceil(8).iso8601(10)  #=> "2010-03-30T05:43:25.0123456800Z"
p t.ceil(9).iso8601(10)  #=> "2010-03-30T05:43:25.0123456790Z"
p t.ceil(10).iso8601(10) #=> "2010-03-30T05:43:25.0123456790Z"

# default argument is 0
p t.ceil.iso8601(10)     #=> "2010-03-30T05:43:26.0000000000Z"

t = Time.utc(1999,12,31, 23,59,59)
p (t + 0.4).ceil.iso8601(3)    #=> "2000-01-01T00:00:00.000Z"
p (t + 0.49).ceil.iso8601(3)   #=> "2000-01-01T00:00:00.000Z"
p (t + 0.5).ceil.iso8601(3)    #=> "2000-01-01T00:00:00.000Z"
p (t + 1.4).ceil.iso8601(3)    #=> "2000-01-01T00:00:01.000Z"
p (t + 1.49).ceil.iso8601(3)   #=> "2000-01-01T00:00:01.000Z"
p (t + 1.5).ceil.iso8601(3)    #=> "2000-01-01T00:00:01.000Z"

t = Time.utc(1999,12,31, 23,59,59)
p (t + 0.123456789).ceil(4).iso8601(6)  #=> "1999-12-31T23:59:59.123500Z"
```

pull request : https://github.com/ruby/ruby/pull/2133



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