How do I convert a decimal number of months to a datetime format?

I have a vector of several decimal values that represent a number of months after Jan 01, 2020.

(e.g. [9.545563 22.212227])

I need to convert these to dates. I'm unsure of how to do this. The ultimate goal of this is to output the dates within an fprintf statement.

(e.g. "Summer 2021/2022 begins on the 3rd of Dec at 3:51:14 and concludes on the 24th of Feb at 22:03:52")

Answer 1

[Walter Roberson]

decmonths = [9.545563 22.212227];

offsets = calmonths(floor(decmonths)) + days(decmonths - floor(decmonths))

offsets = 1x2 calendarDuration array
      9mo 13h 5m 36.643s   1y 10mo 5h 5m 36.413s

base = datetime('Jan-01, 2020', 'InputFormat', 'MMM-dd, yyyy')

base = datetime
   01-Jan-2020

base + offsets

ans = 1x2 datetime array
   01-Oct-2020 13:05:36   01-Nov-2021 05:05:36

Getting the correct output format of "3rd" and "24th" is more difficult. There are no standard routines that will correctly produce that format. (Perhaps there are Java routines for it; I do not know.)

[Jake]

Thank you so much! This is exactly what I needed!

[Voss]

Check that math: 9.5 months after Jan 1 is mid-Oct, not Oct 1.

[Walter Roberson]

@Voss is right, I messed up the fractions of a month. Unfortunately, I am having trouble coming up with the correct formula .

[Jake]

The only way I can think of is manually converting by identifying the length of the month the offset represents.

Answer 2

[Stephen23]

"The only way I can think of is manually converting by identifying the length of the month the offset represents."

Let MATLAB do that for you:

V = [9.545563,22.212227];
W = fix(V);
X = mod(V,1);
A = datetime(2020,W+1,1);
D = A + X.*(datetime(2020,W+2,1)-A)

D = 1x2 datetime array
   17-Oct-2020 21:53:55   07-Nov-2021 08:48:12

For most of those date formats you could download my function DATESTR8601:

https://www.mathworks.com/matlabcentral/fileexchange/34095-date-vector-number-to-iso-8601-date-string

[y,d,m,t] = datestr8601(D(1),'y','d*','mmm','*HMS')

y = '2020'

d = '17th'

m = 'Oct'

t = '21:53:55'

[y,d,m,t] = datestr8601(D(2),'y','d*','mmm','*HMS')

y = '2021'

d = '7th'

m = 'Nov'

t = '08:48:12'