I am looking for a way do linear interpolation between one variable (inv) based the days between another date variable (mth) with the output being a daily time series with interpolated "inv" values. The data series appears to be "quasi" weekly, so the days vary between each observation of "mth", which is proving to be very difficult. I have limited experience wtih R and date calculations, so that's a huge part of my problem. The dataframe is large, but so I created a simple dataframe example below (hopefully a reprex) to illustrate the problem.

library(tidyverse)

library(lubridate)

mth <- c("2019-01-02", "2019-01-09", "2019-01-14", "2019-01-21", "2019-02-03")

mth <- mth %>% as_date(mth)

inv <- c("2450", "2385", "2207", "2115", "1975")

inv <- inv %>% as.numeric(inv)

df <- data.frame(mth, inv)

df

mth inv

1 2019-01-02 2450

2 2019-01-09 2385

3 2019-01-14 2207

4 2019-01-21 2115

5 2019-02-03 1975

I would like the "inv" variable to be interpolated for each day betwwen the "mth" variable creating a daily time series as follows:

mth inv

2019/01/01 2450

2019/01/02 2441

2019/01/03 2450

2019/01/04 2459

2019/01/05 2469

2019/01/06 2478

2019/01/07 2487

2019/01/08 2496

2019/01/09 2385

.

.

.

etc.

Any help you can provide would be greatly appreciated.

CJ